Unlocked s21 Ultra showing as "Unknown Device" and not supporting 5G or Extended Range LTE on My Mobile account

Gamegrrl wrote:

So why does customer support keep leading me to believe this is a "glitch" and a "fixable" thing. And that I should expect it to be remedied within 48 to 72 hours?

I wish I knew.  Support and Customer Care sometimes say things just to get people off of the phone without being more annoyed than when they called.  Your device showing as an unknown device does not restrict it in any way.  If the device is capable of using T-Mobile's implementation of it, it will work as long as the appropriate settings are correct.