I have the same issue but my phone is iPhone 12 Pro Max. At first I though it have the problem with my carrier then I put my sim on Android is working fine. Then, I realized it not thing wrong with my carrier. The problem I have is the combination between T-mobile sim and iPhone12 Pro max (ios14.6). The iOS 14.6 seems not to working with 5G or LTE. Then, I download the newest version of iOS (14.7 beta). Now it works (5g unreliably, LTE reliably). I hope my solution will help your guys.
Always try simple things first (Im sure apple support and Tmobile will make you do them and send you through the loops)
I download the new version on the Apple Beta Software Program.
eice
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