5g not working on 5g 'Bring your own phone'

I bought a Samsung Galaxy S20+ 5G phone from Xfinity Mobile a few months ago. It worked great, however, due to some construction near my home, the cell signal deteriorated to the point where I was dropping nearly ever call. I decided to switch carriers to T-Mobile which has cell towers unobstructed by the construction. I got the unlock code from Xfinity Mobile. When I went to signup at T-Mobile they put their sim card in and rebooted the device but was never challenged for the unlock code. Once switched over I notice I was not getting 5G. We suspected maybe phone was not properly unlocked. I called and verified with Xfinity Mobile that in fact the phone is unlocked. So that may not be an issue. But T-mobile folks say the problem is with Xfinity mobile and 'their ' phone. Has anyone else run into this?  Any resolution?