I have the same issue, ipad will stream fine on wifi, won't stream over tmobile. I also have the tmobile one plus plan. This is the same using the app and through the web.
When I upgraded to the ios 11 beta, the app stopped connecting and syncing, so I began using the website version. This worked fine on both wifi and over tmobile, I could stream live and recorded shows. Then, when ios 11 moved out of beta, and tablo upgraded the app, I was again able to connect and play on wifi using the app as well as website, but nothing would play on cellular.
Android works fine over tmobile, everything works as expected. Ipad app and website both work over wifi. Ipad app and website both load data and sync while on tmobile, but no video will play, live or recorded.
It used to work on tmobile before the new app and before ios 11. I wish I knew if it was tmobile, ios 11, the new app, or some crazy combo. I need my tablo to work again! I am actually considering selling tablo and switching to plex's dvr. I hate to have to do that, I shelled out for the lifetime sub and all!
The only other issue I'm having when viewing remotely on android and my computer, on both tmobile and wifi, any bitrate over 2mbps (other than direct stream) stretches to a strange aspect ratio and a blocky green mess shows up on the bottom of the screen. It's probably unrelated, but I figured I'd bring it up, just in case.