Finally after 3 months we figured out the problem... and seem to have solved it.
One of our HP laptops got a new WIFI Driver update... and now all the Android phone on the network keep their connection!!
Appearantly the ASUS is more sensitive than older routers for the behaviour of bad behaving WIFI clients...
It is still puzzling us why we did not found this solution earlier. It seems that even after the HP laptop was shut down, the problem persisted for a while, hence not giving us the clue that the problem might indeed be there... Dont know... but .. end result is connection stability for all devices