Here, after looking all over and trying different apn info, this is the one that enables the lte, mms, and so far everything. Credits to this user http://forum.xda-developers.com/member.php?u=2102351
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I've already tried the T-Mobile internet and mms apn and the T-Mobile US LTE. They enabled the LTE, but couldn't download or send mms. So far that one above has worked so far. Been using it for about two hours now. Will give an update. Anyone else that sees this, I'm wondering if its possible to "rewrite" my lg g3d850 as a lg g3d851 so i can install tmobile's stock firmware on my unlocked at&t lg g3 and be able to run the phone as if it was a tmobile lg g3, ie, wifi calling, tethering, etc. I got the tethering working through the root and edit values method, just want to know if there's a way to rewrite the entire phone