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

T-Mobile devices don't work that way.  With Android, the Unlocking app needs to meet all of the conditions on the account for it to grant that unlock unless there is an override.  With Apple devices, that request is passed along to Apple, which just takes even longer.  Instead of a few days, expect a few weeks because Apple isn't quick about getting to unlock requests.

Let me tell you where you are wrong and why.  T-Mobile does not lock devices that are unlocked.  Samsung sells devices that come unlock but lock to the first SIM provider you put in it.  That will happen regardless of the carrier SIM you put in it.  That’s a Samsung policy and tactic!  Apple does the same and so do many other manufacturers.  If the device wasn’t sold by T-Mobile, the IMEI has to be given to T-Mobile Support to enter into the system.  Once that is done, you can make an unlock request, if you meet all of the SIM unlock requirements.  Yes, those requirements even apply to devices not sold by T-Mobile!  SIM unlocks can take 3 to 5 business days and sometimes longer, depending on the situation.  Check the T-Mobile Unlock app to see if your permanent SIM unlock request is approved to see if it completes the unlock process.