Check out https://www.cellmapper.net If you zoom in enough, it will tell you what bands each tower supports.
As others have mentioned, band 12 and 71 are T-mobile's sub-1GHz bands, which are best at getting into buildings. Many phones support band 12, but 71 is a little less common (flagship smartphones from the past couple years tend to support it, midrange and low end phones less commonly). But, at least where I live, I generally have no issue with band 4/66 signals indoors, due to density of infrastructure. Pretty much every phone will support band 4 (and band 66 is a superset of that).
As others have mentioned, band 12 and 71 are T-mobile's sub-1GHz bands, which are best at getting into buildings. Many phones support band 12, but 71 is a little less common (flagship smartphones from the past couple years tend to support it, midrange and low end phones less commonly). But, at least where I live, I generally have no issue with band 4/66 signals indoors, due to density of infrastructure. Pretty much every phone will support band 4 (and band 66 is a superset of that).