My Netgear extender and all other Gadget

The last couple days all of my gadgets plus my Netgear Wi-Fi extender have been connecting to my Wi-Fi but then once it connects it says that my internet may not be available but then when I check my Wi-Fi it is running at high speed everything is good on it it's just let the internet work I have tried everything I was on the phone with the cable company for hours I reset it a hundred times it will work for like an hour and then stopped again and now one of my Xboxes it's working but nothing else will connect to it it's saying that the internet is not available