This is what I found to compare.
T-Mobile uses these bands:
GSM850, GSM900, GSM1800, GSM1900, UMTS850 (B5)
UMTS1700/2100 (B4), UMTS1900 (B2), UMTS2100 (B1)
LTE850 (B5), LTE1700/2100 (B4), LTE2600 (B7), LTE1800 (B3), LTE1900 (B2), LTE700 (B12)
The Galaxy C9 is compatible with these bands:
2G bands: GSM 850 / 900 / 1800 / 1900,
3G bands: HSDPA 850 / 900 / 1900 / 2100,
4G bands: LTE2100 (B1), TD-SCDMA2000, TD-SCDMA1900, LTE1800 (B3), LTE2600 (B7), LTE900 (B8), TD-LTE2500 (B41), TD-LTE2600 (B38), TD-LTE2300 (B40), TD-LTE1900 (B39).
The important numbers to look at are T-Mobile using 850/900/1800/1900 and the C9 getting 850/900/1800/1900/2100. That covers what we need for what T-Mobile calls 3G. Note: What T-Mobile calls G3 is called G4 by other companies. Thus what they call 4G is 4G LTE only. So by these numbers, T-Mobile actually gets 4G. IF I understood correctly what I read
As for 4G LTE T-Mobile uses B1/B2/B3/B4/B5 and B12.
Of the T-Mobile bands, the C9 is compatible with only B1 and B3 as well as a bunch of irrelevant ones. This means that if you are in an area where T-Mobile is using B1 or B3, you are in luck and will get 4G LTE. If not, you will have to be happy with 4G.
However, I came here trying to find out how to set it up and which of three slots to put my T-Moblile SIMM card into, so this didn't help me at all.
I hope it helped you!!