Unlocked S21 Not compatible??

I know I've seen a couple similar postings, but none with resolution.  I have an unlocked Galaxy S21.  Per samsung it will work on EVERY U.S. carrier.  It worked on Tmo, Google Fi, AND Visible (Verizon).  Due to it working on the latter, I see no reason why it should be incompatible with Xfinity Mobile, especially seeing as they sell this very model.  I have seen on other postings that people put their old sim in the new phone and it worked fine, other than the system still showing their old phone.  My problem is I don't have an old Xfinity SIM, and they are saying that they can't even send me one due to the "incompatibility" issue, which I think the whole situation seems a little screwy.  Is there any way around this to get a Xfinity SIM?