Original T-Mobile Bands:

2G: Band 2 (1900 MHz)
3G: Band 4 (1700 MHz)
4G LTE: Bands 2 and 4

As of today, there are legacy 2G and 3G networks:

2G: Band 2 (1900 MHz)
3G: Band 4 (1700 MHz) and Band 2 (1900 MHz)

and modern 4G LTE network with support of VoLTE:

4G LTE bands: 2,4,12,66,71

Sometimes instead of Band 4 the phone says that it connected to Band 66 because Band 66 is superset of Band 4. But sometimes it is “real” Band 66. In your case you, probably, still on Band 4 in reality but it is reported as Band 66.

In theory, at some point, you will be able to connect to former Sprint LTE Bands 25,26,41 (may be yes, may be no – depending on phone and starts on the sky).

You can not be on Band 1, you are still on Band 4, but it was reported by mistake as Band 1.