Captive Apple pop up page appears & turns wifi off when I try to access anything???

My extender has worked perfectly for a year, suddenly when I try to access the net With my mobile IOS devices or my desktop Mac I get a non functional captive.apple.com pop up that cuts off the wifi. If I press work offline I get a message saying my extender is not configured, if I continue with this & reconfigure the extender the same thing keeps happening. If I select the modem signal directly everything works perfectly. Tried resets plus lots of other things but nothing seems to work.