Not sure if this is due to my type of plan. But I have had Wifi tethering with tmobile since I switched to them from sprint. However, I switched to a pre-paid (not post-paid) plan and the tethering works. I switched to the pre-paid account about a year ago. I only have the $50/ 500mb plan as I rarely use more than 100mb of data in a month. I strictly use my phone for email,texting and calling. With an occasional tether with my laptop when on the road.
I just tested it, and I can tether my laptop to my Lumia 521 with no issue. I pay nothing extra for tethering.
I am not sure if this had to do with pre-paid vs post paid, but I suspect that is the reason. If there is someone else on the pre-paid tmobile account that can confirm they have tethering enabled, I would like to know. If not, apparently, I have been extremely lucky so far.
And just an FYI to anyone running to switch to the pre-paid account, I called Tmobile a few weeks back to switch to post paid when they switched to no contract pricing, and apparently they cannot move my account to a post paid account (was told this by 2 different tmobile account reps). So I am stuck in a pre-paid account. My phone number is my business, so I do not want to foul that up.