Hi folks, I've seen a couple of post related to this issue and I'm one of it's victims too.
I've justo bought one hEX (RB750Gr3) and two wAP AC (RBwAPG-5HacT2HnD-US).
I have the same problem with both the router and the AP but with different behaviors. With the router I can overcome the problem by connecting my PC to the router using an Ethernet cable. With the AP this approach is not possible and I'm not being able to configure it. If I have no tab opened (ie. QuickSet) it remains connected but a couple of seconds after I open configuration tabs it gets disconnected.
I've read about voltage related issues so I'll try to change how I'm injecting PoE and the other thing I have to try is to connect the AP to the router with no switches inbetween. I'll post later if that fixes the problem.
Meanwhile If anyone has any clue on how to fix this problem I'll be very thankful.