I don’t have any of the new AP One Enterprise yet so this is just a shot in the dark but I assume it needs some minimum firmware version on your Balance Two, you could try updating it.
Also, if you upgraded the AP One E to 3.9.7 RC 2, you would need special firmware on the router. AP Firmware 3.9.7 RC 2
Poking through the release notes for the most recent router firmwares might tell you or you could just try 8.5.4 GA release and see if that helps. If you back up your settings, it’s really easy to reboot these routers back to the previous firmware if you have issues with the new one