Not possible. The chipset in the 3GS uses a different 3G frequency than what t-mobile uses and it cannot be altered. Sorry.
As the title states, is this possible on a jailbroken/unlocked iPhone 3Gs on T-Mobile?
Guess, it should work with 3GS, 3G, and 4G iPhones.
Guess, it should work with 3GS, 3G, and 4G iPhones.