This issue is seeing allot of action on the OnePlus Community page. Apparently, it's a know issue with OnePlus and AT&T not playing well on the 5 and 5T. Maybe one of the AC guys could do a little investigating to see what the issue really is? Right now; if you're using the 5T on AT&T, don't expect much in the way of LTE data speeds.
I'm doing good here on AT&T in Point Pleasant NJ, 52 down, 11up
Well, I dunno if this is the only reason, but while at the AT&T store today I got reminded that their Unlimited Choice plan is limited to 3mb download. We'll see what happens when I upgrade my plan...