T-Mobile uses a lot of different bands, depending on the coverage area. Some areas are on 700 Mhz, others on higher PCS bands (former Sprint bands). I used www.cellmapper.net to see what frequencies were used and confirmed with LTE Discovery app on my phone.
In my case, it was band 66 for a fairly large cell. I've also got a brick house, so that doesn't help. AT&T's 800 Mhz band worked well here but they were jerking around my plan so I switched to Mint.