"I will assume you've ruled out a time/date thing after battery disconnect. Toggle airplane off/on and connect to WiFi.
Last one I had like this was latte damage that I had to desolder the SIM tray to see, it ended up a baseband power ic problem and I gave up. The next one freaked me out with the same behavior so I thought it would be the same. It was jailbroken so hard to see the carrier info. I worried about it for half and hour before I realized it was the silly time/date thing.
