If you bought your phone from a 3rd party source, example, Amazon, Ebay, Black Market or anywhere that wasn't directly from a carrier or directly from Samsung that is why. While you might you have purchased an unlocked phone, you didn't. You bought a phone that was originally a carrier device, and its looking fir the carrier servers for updates.
In @Mujibar screenshot in the area he has highlighted if your carrier doesn't match that of your carrier you have 2 choices, go to a Authorized Samsung service center and have the phone flashed to make it a U1 phone, switch to that carrier
The U in the model name indicates it was originally a carrier-locked device. The phone wasn't properly unlocked by the original carrier and is running that carrier's firmware (AT&T in this case). You won't get any OTA updates unless you insert a SIM from AT&T.
Your other option is to take the phone to an authorized service center and have them flash unlocked firmware. There will be a fee for this service. See this link to find a service center near you:
If its locked to att it means either it wasn't paid off or wasn't unlocked as claimed you can try to unlock it on att site but if its not paid off your not going to get to work on tmobile
I called att and they supposedly unlocked it and gave me a code to give t mobile but it still shows ATT after like 8 hours of them guaranteeing the change. I called t mobile and said they can't do an esim with still showing att. Odd thing is physical Sim card from t mobile does work normally, the problem rn is it's not getting the updates. Very odd situation. I'm going to call again and see what in the world is happening.
