Similar issue - iPhone XR says No Sim after a factory reset. With customer service, we figured it out after a while. First, another shutdown/restart. Some signal strength bars appeared. Then it turns out that the XR is not compatible with 5G in our area. Calls still went to voicemail. Turn on cellular data and it worked.It took a while but at least they figured it out.

was the phone purchased from TMO or brought in from somewhere else? not overly understanding why it wouldnt be compatible with the 5G in your area

Don’t hold your breathe waiting for T-Mobile tech support to solve your issue. I was made to purchase a $140.00 phone because the very phone I was texting on 1 hour before went wanky.

I took it to a T-Mobile store and they said their network doesn’t support it. I told the sales girl that it was on their network 1 hour ago, and she looked at me like I was an idiot and said and I quote “Don’t you understand what I just told you?”

Now I own a $140 brick.