iphone connects to wifi but cannot access internet

My iphone 6s running IOS 15.1 recently stopped being able to access internet, via Wifi and via cellular data. I have tried virtually all the recommended troubleshooting steps, eg:

• trying different wifi networks (my network isn't the problem)
• rebooting router (no change, and other devices are working fine on same network)
• turning phone/airport/wifi off and on
• turning wifi off and trying to use cellular data
• turning VPN off
• kicking out SIM and replacing it
• "forgetting" and re-entering wifi networks
• resetting networks setting
• the only thing I can't do is a complete restore, because that requires me to turn off "Find My", and to do that I need to enter my Apple ID, and to do that I need internet.
• I have tried a couple of dodgy 3rd-party apps which purport to bypass the step above but that didn't work.

Any other ideas? TIA