What you need for T-Mobile Global Data
Simply put you need to be on a POSTPAID plan (not prepaid) and not be on a very old legacy plan...
If you have any form of Data that isn't free - e.g. on your monthly there is some form of PAID data no matter unlimited 1GB, etc, etc you will get Global Data Roaming.
Your US data roaming plan may be 1GB as an example and it has NO effect on your global data roaming - that is unlimited but throttled to a lower speed.