AT&T Fiber blocking ZEN authentication

I've been dealing with this issue for a week now. My company use ZEN (Zero Trust Network) to give us access to their internal websites. Behind the scene, the authentication is done by banyanops. This used to work properly, however, recently this doesn't work on my machine anymore.

Additionally, mobile apps have started showing problem with authentication. Google mobile apps can't authenticate if I'm connected to AT&T Fiber as well. 

If I connect to any other network provider other that AT&T Fiber, everything works fine. 

I've been working with AT&T tech support for a week, and in two occasions, they made changes on their side that solved the issue. However, in both occasions it stopped working the day after. It seems that whatever they changed was reverted overnight.

Has anyone noticed any similar issue?