Is iPhone 6s a 3G device or a 4G one?

Were you Sprint customers when you originally purchased the phones? If so, it may, in fact, be a bit more complicated than just 3G/4G. Phones designed for Sprint and Verizon were slightly different (they have CDMA components as well as GSM). There may be issues when TMobile shuts down the last of Sprint's CDMA network. It may be that is the real problem and it's not being well communicated.

It has 3G and 4G/LTE Capability.

Technically, it should not lose service as it is a VoLTE capable phone.

However... T-Mobile is under no legal obligation to continue to allow it on their network.