An update, I did a ton of research on the internet and I found via Reddit the email address for Neville Ray, CIO of T-Mobile and he responded the other day to email.
Essentially, T-Mobile did not work with Sierra Wireless to certify the EM7455 on their network and thus they do not sell the device nor list it as compatible on their website.
However, he did check and it is compatible and while handsets NEED to be certified on their network to register on Band 12, DATA ONLY devices like the EM7455 do not.
Thus, he said it is an issue of sensitivity, most devices will utilize either Band 4 or Band 2 and fall back to Band 12 when the RSRQ becomes poor around < -13db. He said that my modem is probably staying registered on either Band 4 or Band 2 due to the firmware, but that if I forced it Band 12 the network would allow it to register on Band 12 without restriction.
I have tested his theory and he is correct, in fact I forced my Modem to Band 12 and it did register but because it is only 5MHz of spectrum it was not too fast.