mal: Only 2.5G Despite using T-Mo's suggested APN. I did re-flash SFOS though, a bit after sending the first message.
But to summerize; For non-lte the data apn is epc.tmobile.com * but for LTE devices, fast.t-mobile.com is suggested.
I currently have the data apn fast.t-mobile.com with protocol set to dual (havent tried changing that actually)

* the asterisk for the epc bit is that, in the forums, epc is recomended for MMS, which currently I have set to the default from the SIM