De-T'ed

I was configuring my Nokia G50 to deactivate receiving voicemails. While doing so, I noticed my 5G phone was listed in my home network's area as connected 2.5G service, which it wasn't before. I re-selected 5G, and all seemed well. Until, I turned my phone on again later. Whereas, previously, it always in the upper left corner verified I was a T-Mobile customer,  suddenly it was indicating I was on AT&T. My SIM is T-Mobile, and always was from the time I got the phone, and signed up for T-Mobile. Why is it telling me it is AT&T now?