Thanks for the information but I've already tried all of that. I tested on several Windows 10 devices (two 950XL dual-sim on Insider Redstone build 14342 and a 1020 on the current release 10586.318) with the latest version of the app installed new, and it did the exact same thing on all of them. Meanwhile on an iPhone it works perfectly.
I am pretty sure the app for Windows phones is broken, and it happened about the time that the myatt.att.com website did the color change. Perhaps there were other changes to the website which are causing problems with the app.