https://support.t-mobile.com/docs/DOC-4988
This link explains bands for T-mobile. Basically band 66 is high frequency and gets faster 4G-LTE. Before my old phone did not have band 4 and my newer one after the upgrade did. I was constantly "Buffering" before and this upgrade to a phone with band 4 completely fixed that problem. Short on time but if you can navigate the link it should help.