I get 950 up/down no problem with the standard USG and their switch.
Calling AT&T isn’t going to do you a lot of good unless you’re having the same issue with their modem. They’re not going to support you with the bypass in place.
You’re positive you don’t have IDS/IPS/DPI enabled? Did it only start happening right after the bypass? Test with the bypass and without it within a few mins of one another and see if it goes away.