Best way (in my opinion) is to root that device, flash a new recovery and install a new firmware (ROM). For a Galaxy S2 its fairly easy since it has a huge userbase and is therefore good supported. Here you can find guides for all the necessary steps http://forum.xda-developers.com/galaxy-s2/general
Nice side effect: You get rid of all the unuseful (in my eyes) stuff TMobile bakes into their ROMs.
I personally would suggest you try one of the Android 4.4 ROMs (I personally only know Cyanogenmod and OmniROM), they are running stable, fast and have the latest Android version.
Of course you can also stick to original unbranded Samsung ROMs which include the tethering functionality without limitations.
However, TMobile claims
to implement different optimizations for their net in their ROMs. You would lose these optimizations until you go back to a TMobile branded ROM.