If the problem persists after the factory reset, it's advisable to contact the router manufacturer's support. Provide them with details about the issue and the firmware version you're using. They may be aware of the problem and could provide guidance or offer a solution. If the issue started after a firmware update, consider rolling back to the previous firmware version (if that's supported by your router). Sometimes, new firmware releases may introduce bugs, and rolling back can be a temporary solution until a fixed firmware version is available.

  @Sunshine Thank you, I will wait. Was the issue reproduced?

  @benisfroms Thanks but I've already done all that. As mentioned, firmware rollback is not possible. At least not to the older, working versions. Only TP-Link can answer why rollback to older versions is not possible or blocked.

I've been contacted by support and replied. Meanwhile, I may have some good news. For those users that reported the same problem, please try this:

@toxi @Dobledosis 

Make a backup of your current C80 settings. Consider it a "bad" backup.

Make a note of the C80's default Wi-Fi password and SSID. You will need to connect to it.

Try avoiding a network conflict with your main router: I say this because you will momentarily switch the C80 to Router mode.

Reset all C80 settings to default (go to System / Backup & Restore / Factory Restore)

Connect to your C80's default Wi-Fi network. You may get an internet connection or not, depends on your network configuration and main router. The C80 is in Router mode now.

Whether you get an internet connection or not, proceed below without changing any C80 settings in Router mode.

Only set the time zone, if you can. Do not change any other settings. Now switch to Access Point mode.

After the C80 restarts, check Wi-Fi and the Guest network. If it works, make the settings changes you want in AP mode, and save another backup file, the "good" one.

Let us know what happened.

  @Amiga1000 

Thanks for the information with the procedure from the support.

This procedure also works on the Archer AX12.

  @FalconFX You are welcome. To be honest, I figured it out myself. Support didn't eventually need to do anything.