With Verizon I had issues with WiFi calling on unlocked units until reaching a number of days of the device being on my Verizon plan. I think it was 60days before it was unlocked...?
Moto g7 power refuses wifi calling unless sim removed.
Carriers have a way of blocking features on devices, even ones that are not branded by them just by having their SIM in the device. There was a famous case not long ago, wish I could remember the device, where Verizon had disabled the FM receiver in a device. For some reason one of the big things about the device, or maybe there was a popular app at the time, was the FM receiver. People who switched carriers were able to use the receiver if they switched to another carrier's SIM. The receiver was built into the device but Verizon just locked it.
Not sure how long you've had the device but it could be like my experience with Big Red and just a matter of time before they unlock the feature or it could just be being blocked. Especially with unlocked devices carriers seem to penalize customers for not going with their brand locking. Again using Verizon as an example, they made it so without the Verizon branded software, and again I don't remember the device, customers couldn't Hotspot to other phones. You could Hotspot to a laptop, maybe even a tab, but the protocol needed to allow a phone to phone connection was disabled. This was probably when they charged for Hotspot.
Your best bet is to try and find out T-mo's policy for unlocked devices.