Plug your phone into a computer. On your phone in the notification shade, it should say "Tap for more options" on the plugged in banner. Change it to "software installation"/"Upgrade assistant" (whatever version yours says) and it will install it on the computer. The computer will link to the Samsung servers and download the latest update possible and help you install it regardless of phone status. This is fully dependent on model and region, but since you're on ATT, that's American and should work for you.
AT&T provided the updates from their own servers, and based on complaints the last few years on AT&T's community forum, many people don't get updates. Most do, but a significant number of people have never received an update. Customer reps don't have a solution except to regurgitate the same process everyone already tried, and no one higher up at AT&T even bothers to figure out the problem.
If you're not getting the update now - and out of curiosity, what version and security patch are you on now?- you probably aren't ever getting the update.
Oh, and the One UI 2 update came out last March - that was the Android 10 update. Just a few weeks ago was the July 1 Security update that included the minor One UI 2.1 update.