iPhone 14 Pro Max connects to WiFi but will not communicate over the Internet

My iPhone 14 Pro Max will connect to any WiFi, but will not communicate on the Internet over WiFi or 3G, 4G or 5G.

I have tried various routers and WiFi networks. It connects, but still won't work.

I have upgraded iOS (via an iMac) from 18.1.1 to 18.2. Still won't work.

I have tried most things I can think of. Still won't work.

Has anyone else found the root cause of this problem? I know it has been asked here before.