I had the problem with the iPhone 11 Pro Max after an iOS 14 Update. i tried everything including full file restore and new sim. After toggling flight mode it was working for a while and then stuck with edge/2G and nothing was loading anymore. Sometimes it was going back to 4G and sometimes it stuck until restart or toggling flight mode. Apple did not help. Genius Bar refused any help because it was already a few months out of warranty. The only thing Apple offered me was 390€ for a 1360€ iPhone as a trade in for my new iPhone 13 Pro Max. I had to take it because it had no other chance. As long as I don’t know what is wrong with the phone I couldn’t sell it to someone else…