It can often take some time for the carrier to recognize a device and add it to the update queue when you first activate it.
If you don't want to wait, I'd try to load the new firmware using Samsung's Smart Switch. That will identify your device, then grab and install the latest correct firmware. Some time ago, however, I read that AT&T has done something to prevent some of its devices from being updated with Smart Switch. If that's the case and Smart Switch doesn't work for you, that leaves using ODIN (Samsung's maintenance utility) and doing it yourself.
You have to be very careful because it is also very easy to brick your phone if the process does not complete correctly. Understand how the process works, why it works as it does, and how to recover if things go wrong. You will also lose everything on your phone. So make sure that all of your photos, docs, music, etc are backed up. If the process works, your phone will be like it first came from the box with the latest Android 7 (that's the latest Android 7 as released by AT&T, not the no-fooling latest 7.1.x from Google).
The actual process for reflashing the firmware is just a matter of downloading ODIN and finding the right download package for your device. You can't mix and match here. Your phone won't work correctly with the incorrect firmware. This tutorial from one of the mods here is a good place to start.
[Guide][Samsung] How to flash Stock ROM via ODIN with the New Interface
Good luck.