I purchased a new S20 (SM-G988B). I swapped the sim card over from my old phone and promptly got a text message noting that the "network is being upgraded and the cell phone you tried to activate would no longer work".

I reset the networking settings, manually added the APNs, did a full reset of the phone, etc. Still had no service.

I called the ATT support line to try to activate it, and after failing to achieve success they eventually told me that the sim card was incompatible with the new phone.

I then took it into an ATT store, they tried to get it to work and couldn't, they then activated a new sim card and tried to get that to work but failed and eventually told me that the device was not supported. That when they were trying to activate it the system would just error when the IMEI was entered, and the support person they called (they spent half an hour on the phone with support while I was in the store) said the same thing.

I just tried again today with the sim card (that works fine in my old phone) and briefly had it show 4g in the status bar and the status page showing that it was in service, email notifications started coming in, but a few seconds later it received the same text as last time stating that the phone is incompatible, and all service stopped.

Obviously the frequencies are not an issue - it is compatible with the network (otherwise it would not have ever shown as registered even briefly). There just appears to be something within the ATT system that is not allowing this phone to connect.

Any ideas on what the issue or solution is?