I have a new Airport Extreme (tall, latest model) and a previous gen flat base station. Both were working fine until about 2 weeks ago. Now when I go into the Airport Utility on Mac or iPad, the main Base Station Extreme that is plugged into the Comcast Router via Ethernet cable shows Device Not Found. But I am connected to the network I setup on this device. The Older Model Airport extreme stopped connecting and flashes yellow and even with a hard reset it now doesn't even show up in the Airport Utility. Haven't had any issues with these for 2+ years and nothing changed on the network side - just newer iOS9 and El Capitan on the devices.