Originally Posted by LordHamster

In China/Taiwan/HK, the roaming partners can be problematic mostly because of the frequency bands those partners use. For example:
In China, T-Mo partners with China Mobile now (used to be CHina Unicom). China Mobile's LTE bands are incompatible with most phones T-Mobile sells, meaning you'll end up roaming on whatever ancient protocol their LTE network falls back to.

Your best bet, is to buy a Verizon, Sprint or "Sim Free" model iPhone, then using that with your T-Mo sim. You will have better global roaming that using an actual T-Mo iPhone.