You can run these commands to confirm for yourself (as in this tutorial):
mmcli -L to get your modem number (example: “0”)
Then run:
mmcli --modem=0 (change “0” to match your own modem number)
Now look for the line:
operator id:
If the numbers displayed there are associated with T-mobile (e.g. “310260” ), then it’s using T-mobile’s network.
See also: https://mcc-mnc-list.com/list