AC1200 range extender taking over wifi network

My AC1200 easily connects to my wifi network and the signal strength is very good. Unfortunately within a few hours of connecting the extender all devices connected are kicked off the wifi and (althought they look like they are connected there is no connection) I have to turn off the extender and reboot the router to get the wifi back up and running. I have had our internet provider out several times to test the connection and there is nothing wrong with our fibre op connection or the wifi signal strength or stablility.