I've had a really good experience with my OnePlus 3T, even after having to spend some time with AT&T support right after I got it so they could update their system so I could get 4G LTE.
The only ongoing problem I'm having is that depending on my location, my service will drop from LTE or LET+ to H, H+, or no service. We live and work in the suburbs of a major metro area, and AT&T's map shows us blanketed in 4G LTE. When my service level drops, my wife (who also has at&t) is usually still getting LTE on her Galaxy S8. I always get 4G at work, but it is really hit or miss (usually miss) at home.
What seems odd to me is that some days at the house I'll be getting 4G/LTE just fine, and others it will be H+ or no service.
My suspicion is that the OnePlus 3T doesn't support all the LTE frequencies that AT&T uses, which is why my LTE service is intermittent. Has anyone else with this phone had similiar problems?
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.