After a clean wipe and restore, I get a modem firmware finally and I get att signal (brand new sim after store visit), works for about 18 hours then tries to update cellular again and I am back where the phone uses WiFi only. I guess I could just continue these long painful restores every 18 hours so I can use my phone as a phone.

Just got back from AT&T, got a new sim, finally got a signal, got home and the dreaded no cell, you need to update blah blah blah. Reinstalled 14.2 after wiping completely and after restoring my backup, behold, no signal but wifi. I know it's a baseband problem, if you look at modem firmware under general in settings, it is BLANK. So, something is corrupting the baseband firmware. All this is on iPhone 7, I can put my sim in my iPhone 6 and have no problems but it is on 12.4.9.