@Joseph-TP It is a RT-N66U - not exactly top of the line... It is (as mentioned previously) set to AP mode.

@Joseph-TP I do not have a modem - my Archer is connected directly to a RJ45-plug in the wall, but otherwise yes. Yes, it did start after the firmware upgrade, and yes, it worked reliably before....

Okay, I have made a bit of a discovery here... I have an old ASUS router that is set up as an access point, and it I remove the cable to that from the Archer, after 10 seconds or so, the WAN-light...

@showview Yes - I have the same router, same firmware (which I did manage to find on https://www.tp-link.com/en/support/download/archer-ax55-pro/v1/#Firmware) and the same problem! I did not flash it...

@gtbiss I hate to say it, but at this point it looks like TPL doesn't care at all. I doubt it'll ever be fixed...