This is a warranty issue. Back to any ATT USA store and make your claim so they can fix it.
I have a Samsung Galaxy S6 from AT&T USA that got unlocked. I've used it for several months with no problem, but all of the sudden, after a reboot I would not get mobile service, and IMEI has changed to 350000000000006. Can you please tell how can I fix the IMEI?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic