I've used the data pass in countries where I'm nowhere near a place where I can pick up a SIM card.
It really depends on which carriers you're roaming on and what kind of agreement they have with T-Mobile. Some carriers will work fine, others may deprioritize the off-network SIM, so even though you may get full speeds, it may not be sustained.
Last edited by diburning; Oct 6, 2017 at 9:16 am