Will I receive Android OTA updates after switching carriers?
I have a Galaxy S8 from Verizon and I switched to an MVNO which piggybacks on T-Mobile's network.  With the new carrier, the phone does not recognize the service and does not perform an update check.  However, after inserting the original SIM card, Verizon does allow the system update to be checked, the update to be downloaded, and installed.  I then have to switch the SIM card back to my MVNO.
Perhaps it works the same way with AT&amp;T phones. May be worth a try.