To recap:
You updated from 8.0 to 8.1. When you try and turn on internet sharing, you get a message asking you to add the feature (useless: anyone with T-Mobile and data has an internet sharing plan right off the bat, it isn't optional). When you agree, and choose to upgrade, it sends you to T-Mobile's upsell web page, that informs you that data isn't available?
And in 8.0, internet sharing worked?
If so, you've got the same problem I have with the 925. If you're using an international phone, than you're having almost the exact same problem. In which case, unfortunately, there isn't an actual solution. Your only "fix" is to downgrade to WP 8.0 using the Nokia Recovery Tool--which will mean sacrificing the new update functions (Cortana, etc.).
However--keep in mind, certain later updates (8.1.1, some call them) will break the ability of Recovery Tool 1.4.3 to do that. Once you update it, you're not going to downgrade until Nokia updates their recovery tool (which they may or may not). Unfortunately, GDR1 (as I believe it was actually called) doesn't fix the problem (it does give you nifty Live Tools) though.
If you look under my threads, you can see my thread for the same issue in 925, and see if it sounds similar. I hope you don't have the same issue, but if you do, you're sort of boned, frankly. I blame myself for thinking that GDR1 would fix the problem (it didn't) and believing Nokia's tech team when they said it was safe to upgrade, the Access point was the issue (they were wrong).
I hope it's not the same problem, but it sounds like it might be.