I have multiple clients using ATT with no problem but they are all using LTE-A units so I wonder if that is your problem if you’re using a regular LTE unit. I’ve been around the block with ATT regarding APN’s and this seems variable. Try manually entering “nextgenphone” with an LTE-A unit and also “broadband”. One rep told me it doesn’t matter, but sometimes it does. Throttling also seems variable, sometimes quick and other times not until 120Gb. An ATT rep told me just this week that they will throttle you quicker in more congested urban areas.
I posted this earlier, but here’s a much more cost effective source for that antenna. Get a 2nd one and their splitter/combiner to phase two antennas if you need the gain:
http://www.altelix.com/Wide-Band-Directional-Antenna-50-Ohm-12dB-2G-3G-4G-p/al0727g12-nf.htm
Once again, a built-in speed test would be nice for testing these problem installations.
Last, suggest you read this: Extreme range BR1 - #7 by Rexymav