I believe T-Mobile's LTE runs all at 1700/2100mhz (AWS). Their HSPA (mislabeled 4G) runs at 1900, 1700/2100, and roams to AT&T at 850 (due to the failed merger).

The higher the frequency, the harder it is for the signal to work indoors. I pretty much rarely see LTE indoors at work or around businesses near work. But HSPA holds up solid.

T-Mobile does have a cell signal repeater they can provide if you have signal issues. Not sure the hoops you have to go through to get one for free though. It's based on the Cel-Fi product. This is an actual repeater, not a microcell that uses an internet connection.