AT&T not working

[Shubadub]

Hello!

I recently purchased the US add on package deal, and am now realizing that whenever my phone is connected to AT&T, my data does not work. My friend is experiencing similar issues where, her data works but her call does not work. Whenever my phone automatically connects to T-Mobile, data works fine. The reason why I am wondering is because AT&T is much more prevalent in the areas we are. 
I have tried to contact Public Mobile but nothing seems to load (hence why I need data…).

Thanks in advance! 

[Shubadub]

Going to try this, thank you for the quick reply! 

[BKNS27]

@Shubadub 

Try resetting your network settings on the phone then manually connect to AT&T.

[Phil_Adelphus]

@Shubadub   Is your phone set to 5G?  5G is reported  not to work well on AT&T so set it to LTE.  Regarding your friend's phone, when voice calling is not working it suggests probably the phone is not VoLTE enabled in which case try dropping to 2G and connecting to T-Mobile.  The US has shut down its 3G network which can make roaming a little tricky for some phones.