You've probably already checked this but do you have a setting on your phone for network mode? I have a different phone altogether and just kinda stumbled across your question. Anyway on my device in the settings at data usage > more > mobile networks > network mode > LTE. Your setup may be a little different but its just a thought.
If not that, I'd wonder if you are getting the advertised coverage. Here, TMO claims 4G where I'm at but you're lucky to even get Edge in reality. Different carrier but perhaps a similar kinda deal. Hope that's not the case.