At the moment T-Mobile only broadcasts 5G on n71, n41 and mmWave (very isolated deployment).
In the future there will likely be n25/2 and n66, but probably not this year.
As others have said, the 5G icon shows if the band you are connected to is enabled to serve as an anchor band for 5G. At the moment, B2 and B66 serve as potential anchor bands. B12 may be an anchor band in some areas. B71 is not used as an anchor band at this time. The only way to know for sure if you are actually on a 5G band is through an engineering screen or one of the signal apps mentioned.