Here’s a simple solution:

Don’t use the T-Mobile connection settings. You might also have to have a different SIM. I don’t have an active T-Mobile SIM, but the settings may stay after configuration. You may be able to change the setting, then go back to your T-Mobile SIM.

I have mine SIM unlocked as well. That might make a difference. And I have a Solavei SIM.

Go to the apps and select Connection Setup. I will say Multiple Connection settings were found. Select something that ISN’T T-Mobile. I picked Roam Mobility.

After this you should be able to turn on Internet Sharing.

This works on my Solavei SIM. I did it for experimental purposes. I’m just testing the Windows Phone HTC 8X model for this type of thing before I’m back to my Nexus 4.

Sign up for service with us, and I’ll pay for your SIM card if you mention this blog post.

Solavei doesn’t allow tethering per the contract. The term hotspot is not used, but they will tell you it’s disallowed. Your call.