Hi
well to simplify this, let's say you have 2GB of data per month, it doesn't matter what speed is used either due to your phone capability or due to what's available at a certain location you are still using data and it will count against your allotment, the one thing about T-Mobile is once you hit your limit which is the 2GB in this example, the speed will be throttled to 2G speed well not great but better than nothing and free of charge till your next billing cycle that's why they call it unlimited data since you still can use even after reaching your limit, as I said it will be slow but better than nothing
if you're a heavy data user then you might need to consider the 80$ plan that gives you the unlimited everything plus 5gb of tethering