Im pretty sure its carrier related.
Okay so first thing is first let this be known that this was not created by me. I'm just sharing along since I found a fix. All you have to do is Google up "universal boost mobile APN fix for d2lte" its a mod created by mopedryder. All credit...forum.xda-developers.com
Not sure how it could be, when everyone so far has stated their APN settings were correct already (or were adjusted) and still have connection issues. Same on my end. Never had an issue with T-Mobile until this ROM.