I purchased the TP-Link MR6400 router (v.5 hardware revision) to manage an LTE connection provided by the Iliad operator, in Italy. The price was about 67 € in January 2023.

Following the OpenWRT Firmware Selector and choosing the option TFTP-RECOVERY, I downloaded the firmware file openwrt-22.03.3-ramips-mt76x8-tplink_tl-mr6400- v5-squashfs-tftp-recovery.bin.

• Configure your PC as a TFTP server. On a Debian GNU/Linux PC you can:
  1. Install the tftpd Debian package.
  2. Verify in /etc/inetd.conf that the document root of the service is /srv/tftp/.
  3. Rename and copy the firmware file to /srv/tftp/tp_recovery.bin, with mode 644.
  4. Configure the Ethernet interface of the PC with address 192.168.0.225/255.255.255.0.
  5. Restart the inetd service with systemctl restart inetd.service.
• Connect the TFTP server with an Ethernet cable to the LAN1 port of the router.
• Power on the device while holding the reset button (waiting 8 seconds should suffice).
• Check the /var/log/syslog of the TFTP server: the router will request the tp_recovery.bin file and and install it.

Once the firmware is installed, the router will reboot and it will start with an IP address of 192.168.1.1 on the LAN ports 1 to 3. You can connect using HTTP or SSH, default setting have no password for the root user.

The default OpenWRT configuration uses the LAN port 4 as WAN (i.e. connection to the Internet). If a DHCP server is found on the LAN, the IP address, DNS and default gateway will be aquired automatically.

Connect to the router via SSH and install the following packages (I use OpenVPN to make a VPN between two offices and I use tcpdump as a debug tool):

opkg update
opkg install luci-proto-qmi
opkg install openvpn
opkg install luci-app-openvpn
opkg install tcpdump-mini

The repository used during the installation is the following:
https://downloads.openwrt.org/releases/22.03.3/packages/mipsel_24kc/.

After the installation of the above packages, this is the free space on the filesystems:

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 3840      3840         0 100% /rom
tmpfs                    28800       976     27824   3% /tmp
tmpfs                    28800        60     28740   0% /tmp/root
tmpfs                      512         0       512   0% /dev
/dev/mtdblock4            1920       912      1008  48% /overlay
overlayfs:/overlay        1920       912      1008  48% /

Using the stock uqmi package provided by OpenWRT 22.03.2 I was not able to bring up the LTE interface. The error message shown by logread was:

daemon.notice netifd: lte4g (2329): Command failed:
    ubus call network.interface notify_proto
    { "action": 0, "link-up": false, "keep": false, "interface": "lte4g" }
    (Permission denied)
daemon.notice netifd: Interface 'lte4g' is now down
daemon.notice netifd: Interface 'lte4g' is setting up now
daemon.notice netifd: lte4g (2343): Waiting for SIM initialization

I downloaded a custom uqmi packages for OpenWRT 22.03.2 here: uqmi_2022-11-29-0.10_mipsel_24kc.ipk.

The uqmi depends upon libubox and libblobmsg-json, these are the exact versions that satisfied the dependency in my installation:

Now the logread output is OK, except that IPv6 is not negotiated (may be a fault by the Iliad provider):

kern.info kernel:  qmi_wwan 1-1:1.4: cdc-wdm0: USB WDM device
kern.info kernel:  qmi_wwan 1-1:1.4 wwan0:
                   register 'qmi_wwan' at usb-101c0000.ehci-1,
                   WWAN/QMI device, 1a:45:b9:77:47:66
kern.info kernel:  usbcore: registered new interface driver qmi_wwan
kern.info kernel:  usbcore: registered new interface driver option
kern.info kernel:  usbserial: USB Serial support registered for GSM modem (1-port)
daemon.notice netifd: Interface 'lte4g' is setting up now
daemon.notice netifd: lte4g (1900): PINcode disabled
daemon.notice netifd: lte4g (1900): Data format set to raw-ip
daemon.notice netifd: lte4g (1900): Default profile: 1
daemon.notice netifd: lte4g (1900): Change default profile
daemon.notice netifd: lte4g (1900):  apn:  to iliad
daemon.notice netifd: lte4g (1900):  authentication: none to both
daemon.notice netifd: lte4g (1900): Airplane mode off
daemon.notice netifd: lte4g (1900):  registered on 22250
daemon.notice netifd: lte4g (1900): Registered to Iliad on LTE
daemon.notice netifd: lte4g (1900): Connected with IPv4
daemon.notice netifd: lte4g (1900): Unable to connect with IPv6
daemon.notice netifd: lte4g (1900): Setting up wwan0
daemon.notice netifd: Interface 'lte4g' is now up

Edit the /etc/config/network file and add a section like this:

config interface 'lte4g'
        option proto 'qmi'
        option apn 'iliad'     
        option autoconnect '1'
        option device '/dev/cdc-wdm0'
        option delay '1'
        option auth 'both'
        option pdptype 'ipv4v6'
        option default_profile '1'

You can now verify that the new interface is shown into the LuCI web interface, into the Interfaces page. Click over LTE4G and check the General Settings:

• Protocol: QMI Cellular
• Bring on boot: [x]
• Modem device: /dev/cdc-wdm0
• APN: iliad
• PIN: <blank>
• Authentication Type: PAP/CHAP
• PAP/CHAP username: <blank>
• PAP/CHAP password: <blank>
• PDP Type: IPv4/IPv6

Once the network device is created, you must add it to the WAN firewall zone: Interfaces ⇒ LTE4G ⇒ Firewall Settings ⇒ Create/Assign firewall-zone: WAN.

When the LTE connection is established you will see an wwan0 interface up and configured. It is possibile to query the status of the connection using the uqmi tool. SNR (Signal to Noise Ratio) and RSSI (Received Signal Strength Indication) should both give you an indication of your signal strength:

uqmi --device=/dev/cdc-wdm0 --get-signal-info
{
        "type": "lte",
        "rssi": -58,
        "rsrq": -8,
        "rsrp": -94,
        "snr": 16.600000
}

The TL-MR6400 router has seven LEDs:

Once you installed the OpenWRT operating system, you can control them accessing the pseudo files into the /sys/class/leds/ directory. Each LED have its name (actually a subdirectory):

You can manually test the on/off operation sending a specific string to the trigger pseudo file:

echo "default-on" > /sys/class/leds/white:signal1/trigger
echo "none"       > /sys/class/leds/white:signal1/trigger

Reading the same pseudo file you can get the options supperted by the trigger, the current state is highlighted by the square brackets:

cat /sys/class/leds/white:wlan/trigger 
[none] switch0 timer heartbeat default-on netdev usbport phy0rx phy0tx phy0assoc phy0radio phy0tpt

The trigger netdev can be used to connect the LED status to a network device status (link, activity, etc.). It seems to be integrated into OpenWRT 22.03.3; the package kmod-ledtrig-netdev does not exists and it is not actually required.

The default LED configuration is found into the /etc/config/system file. It binds the LAN LED to the carrier status of ports 1 to 3 of switch0 and the WAN LED to the carrier status of port 3 of switch0 (the port_mask option is a binary mask of ports position: 0x07 = 0b0111, 0x08 = 0b1000):

config led 'led_lan'
        option name 'lan'
        option sysfs 'white:lan'
        option trigger 'switch0'
        option port_mask '0x07'

config led 'led_wan'
        option name 'wan'
        option sysfs 'white:wan'
        option trigger 'switch0'
        option port_mask '0x08'

I want to use alle the four ports as LAN ports and I want to bind the WAN LED to the LTE connection, so I have changed the configuration as follow:

config led 'led_lan'
        option name 'lan'
        option sysfs 'white:lan'
        option trigger 'switch0'
        option port_mask '0x0f'

config led 'led_wan'
        option name 'wan'
        option sysfs 'white:wan'
        option trigger 'netdev'
        option dev 'wwan0'
        option mode 'link tx'
        
config led 'led_wifi'
        option name 'wifi'
        option sysfs 'white:wlan'
        option trigger 'phy0tpt'

For the WiFi LED the phy0tpt trigger seems to be the most adequate, I tried to use the phy0radio, hoping that it will reflect the on/off status of the radio transmitter, but it is not the case: sometimes the LED remained off even if the WiFi signal was active.

The uqmi_d.sh daemon provided by the custom uqmi package contains an hook to update the LEDs of the LTE connection. Every 30 seconds it calls the script /usr/bin/uqmi_led.sh, which is not provided, you have to create your own. This is mine:

#!/bin/sh
 
RSSI="$1"    
if ! echo "$RSSI" | egrep -q '^[+-]?\d+$'; then
        echo "Usage: uqmi_led.sh [RSSI]"         
        exit                            
fi          
echo "$RSSI" > '/tmp/lte-rssi'
 
LED1=$(readlink -f /sys/class/leds/white:signal1)
LED2=$(readlink -f /sys/class/leds/white:signal2)
LED3=$(readlink -f /sys/class/leds/white:signal3)
 
if [ "$RSSI" -eq -200 ]; then                    
        echo none > $LED1/trigger
        echo none > $LED2/trigger
        echo none > $LED3/trigger
elif [ "$RSSI" -le -90 ]; then   
        echo default-on > $LED1/trigger
        echo none > $LED2/trigger      
        echo none > $LED3/trigger      
elif [ "$RSSI" -le -60 ]; then   
        echo default-on > $LED1/trigger
        echo default-on > $LED2/trigger
        echo none > $LED3/trigger      
else                                   
        echo default-on > $LED1/trigger
        echo default-on > $LED2/trigger
        echo default-on > $LED3/trigger
fi                                     

The script is based on the uqmi_led.sh example, but I added some syntax check and the creation of a cache file where to store the RSSI (Received Signal Strength Indication) value. That cached value is used to make the value available via the SNMP service (see below).

Installing the full snmpd daemon and the relative LuCI web interface requires about 548 kb of storage space on the /overlay partition. The required packages are:

opkg install snmpd
opkg install luci-app-snmpd

You have to restart the LuCI service to see the new Services ⇒ SNMP page.

Unfortunately the web interface is rather useless: to add an extend OID it is necessary to edit manually the configuration file. The following example adds an OID to read the current RSSI value of the LTE connection; add this section to /etc/config/snmpd:

config extend
        option name 'lte_rssi'
        option prog '/usr/local/bin/snmp-lte-rssi'

Restart the SNMP service with /etc/init.d/snmpd restart and verify that the configuration file /etc/snmp/snmpd.conf created on the fly is correct.

The script that will serve the OID value is /usr/local/bin/snmp-lte-rssi:

#!/bin/ash
# If the file RSSI is fresh, return its content, otherwise return an empty line.
RSSI="/tmp/lte-rssi"
if [ -f "$RSSI" ]; then
        NOW="$(date +%s)"
        FILE_MTIME="$(date +%s -r "$RSSI")"
        FILE_AGE="$((NOW - FILE_MTIME))"
        if [ "$FILE_AGE" -le "60" ]; then
                cat "$RSSI"
                exit
        fi
fi
echo

It is possibile to query the SNMP service using the snmpwalk command line tool:

snmpwalk -v 2c -c public 192.168.1.1 'NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."lte_rssi"'
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."lte_rssi" = STRING: -65

I was not able to instal the wakeonlan package because it depends upon Perl (not enough space on flash memory). Fortunately I was able to install etherwake, which requires only 4 kb.

To upgrade from OpenWRT 22.03.3 to 22.05.0 we used the sysupgrade option, which preserves the existing configuration, at least partially.

Download the new firmware openwrt-23.05.0-ramips-mt76x8-tplink_tl-mr6400-v5-squashfs-sysupgrade.bin to your PC. Connect the PC to one of the Ethernet ports of the router and point your browser to the LuCI web interface. Click the System ⇒ Backup / Flash Firmware page and then upload the firmware file to the router by clicking Flash new firmware image.

Once the router reboots, you can access it via SSH or LuCI web, the network configuration should have been preserved.

Beware that all the extra packages must be installed again, notably the uqmi package, required to drive the LTE modem for internet access, must be upgraded to version uqmi_2022-11-29-0.11_mipsel_24kc.ipk. That version is not included into the official repository and thus must be downloaded from the GitHub repository.

The repository roots are:

• https://downloads.openwrt.org/releases/23.05.0/targets/ramips/mt76x8/
• https://downloads.openwrt.org/releases/23.05.0/packages/mipsel_24kc/

This is the list of all the extra packages installed:

• OpenWRT - TP-Link TL-MR6400 v5
• Customized OpenWRT uqmi package by Mrhaav
• OpenWRT - LED Configuration
• [TL-MR6400 v5] LTE “disconnect” after six hours