The device does not announce itself as an Android device on the USB ports. Running the adb tools on the attached GNU/Linux PC does not show any:

adb  devices
List of devices attached

1. Unplug the power from the device.
2. Connect a screen to the HDMI socket.
3. Connect an USB keyboard and/or an USB mouse (mouse required for some recovery, e.g. TWRP).
4. (Optional) insert the SD card with the recovery.img on its root directory (FAT32 format).
5. Insert a toothpick into the A/V hole, and push it untill you feel the click of a microswitch.
6. Keeping the toothpick pressed, insert the power plug.
7. Wait until you see the recovery screen (about 15 seconds).