I wonder if this has anything to do with them removing SA from this build’s carrier profile.I'm also on T-Mobile on an iPhone XR and installed the 14.5 Public beta 5. My cellular data is not working at all which means no internet, no iMessage. However, the signal bars show up with the LTE logo. I am able to connect via wifi without any issues. Is there a way to downgrade to a previous beta release?
Have you tried Reset Network Settings?