Thumbs up & happy to hear that OP's problem solved & data is working overseas as promised by T-Mo, and, at reasonable speed.

It's no fun to be out of the country & head banging against the device, feeling helpless & tech. support are scratching heads with you.

We've had similar issues with users coming to the Nexus 5 forum on XDA-Developers & the knowledge based were being shared, and solutions found. I threw out those as possible settings to check for, not all of them are accessible and/or applicable to different devices & on different platforms - but that fast.t-mobile.com change/addition for the APN was among the keys to resolving the connectivity issues. (epc.t-mobile.com was a common one & older devices using voicestream.com) Trial & error and process of elimination were applied, flipping between IPv4 and IPv6 has more to do with tethering.