Using ADB locally on device (loopback) shows no devices
I believe your device might have rebooted. To initially get your phone into tcp, you must have ran adb tcpip 5555 from a computer. But that doesn't last forever. If your phone gets rebooted, then you will have to run adb tcpip 5555 from a computer again.
I am not sure what you meant when you said:
Trying to restart adb(d) in TCP mode is unsuccessful (even though this was not necessary in the working situation). adb tcpip 5555 returns error: device not found.
Did you run that from a computer? If it was from a computer then I'm not sure what the problem is there. But if it was from the phone, then it can't find the device to change to tcp.
To fix this, adb tcpip 5555 from a computer should work.