Original Carrier List

Trying to switch from T-mobile to Xfinity Mobile but T-mobile is not an option on the original carrier list.Tried reaching out to support via the Xfinity Assistant and keep insisting I provide them with all my information. I simply just want to know why I cannot select T-mobile and complete my switch using the website. Would love if they could troubleshoot and fix this issue so that I can continue the process of switching. This is extremely frustrating and is definitely making me rethink the decision to join Xfinity Mobile.