I have the same problem exactly with the same contoller 3600 and software version at my place of work, Even tough the iphone 7 do not disconnect when locked at home with 3Com router.
When i made debugg it showed the client disassociate then reassociate with every lock or sleep from the phone.
Please need to know what option exactly in the controller so i can keep the iphone connected all the time and do not miss any notifications.