Can't connect to multiple devices
I have a fourth-gen Airport Extreme which was working perfectly fine until yesterday, when I attempted to extend the network using an old Express. At one point, I accidentally selected the Extreme as the network extender and it went haywire. Since then, I have factory reset it three times, called my ISP, tried every other trick in the book, but it still says it can't get an IP or DNS address and I can't get internet through it on more than one device at a time. For example, my iMac is working perfectly on my wifi but while I can access the Airport Utility on my network via my iPhone, Safari just spins forever and can't load anything. Additionally, my other peripherals can't get internet either -- including my Apple TV, which also connects to the network but cannot connect to Netflix, etc.
is my Extreme busted? HELP!
AirPort Extreme 802.11n (4th Gen)