Just spoken to T-Mobile and my phone is blocked from being used as a modem.
Not necessarily a problem. Depends how it’s being ‘blocked’.
On Orange android phones I’ve had it has been done by means of using a specific APN on the phone for modem/broadband, and a separate one for normal stuff.
They also make it so you cant delete or modify the APNs.
However, if you just create a new APN with no ‘type’ and select that for use, the phone uses it for everything and tethering works fine.
If that doesn’t work, you can of course replace the OS software with something custom that hasn’t been fiddled with by your operator.