AT&T "mobile network disconnected" error in new HTC U Ultra phone

You have been found out by AT&T's IMEI sweep.

AT&T will not allow phones that won't be supported after the 3G shut down early next year to be newly activated on their service.  If you try, it might work at first, but they will soon deactivate the SIM, just as they did with your phone.

I know you will say "but it is a 4G LTE phone".  Sure, but it does not support AT&T's VoLTE (what they call "HD Voice").  And that's a totally different thing. The HTC U Ultra drops to 3G for voice calls and texts, so when 3G is turned off, so is that phone's ability to make a call.

Yes, your previous phone worked because you probably had it before they quit activating those future unsupported phones.  Had you kept it, you would have gotten a text from AT&T telling you that your phone will be cut off when the 3G shut down happens.

https://www.att.com/idpassets/images/support/wireless/Devices-Working-on-ATT-Network.pdf 

The list linked above tells you which phones will still work after the shutdown in Feb of 2022.  Notice no HTC U Ultra.  Only 5 HTC phones, a couple of Desires and some Ones are on that list. 

Thank you very much.  I really appreciate your detailed and very helpful response. 

I am clearly doomed...  I love my HTC U Ultra, but it looks as though it's now just an expensive paperweight!

I could almost suggest T-Mobile, as they are much more BYOD friendly than AT&T ever was, but I fear they will be doing the same thing soon enough.  Might get a bit more use out of it, though.  I'd ask around in their support forums, just to be sure.

I hear rumblings of their impending 3G shut down, but I don't follow them enough to know any certainties.