First please confirm your devices "Service Provider SW ver." which can be found in Settings > About Phone > Software Information. Scroll down till you see this and make sure your device has ATT/ATT/ATT. The most important is the third set (--/--/ATT) as this identifies the carrier a Samsung device was from the factory. The 1st & 2nd set (ATT/ATT/--) are identifying the device's firmware installed. If you do have a factory AT&T Galaxy S10+ and have a active AT&T sim card installed then it should pull all OTA software updates from AT&T.