My AirPort Extreme (the 802.11ac 'tower' type) has recently stopped being accessible from AirPort Utility. It still works just fine otherwise, both as a wireless router and for Time Machine (with an attached USB hard drive).
If I restart it by unplugging and plugging the power back in it is again accessible from AirPort Utility, but only for a short while. I've tried a full factory defaults reset and set it up again, but the problem continues. It's accessible for a while, then not.
It shows up in AirPort Utility on macOS, but when I click on its icon it prompts me for a password. When enter the password it thinks for a while and then does nothing. It just keeps asking for the password, which I know I'm entering correctly. Also, when it's working normally for the short time after a restart, it doesn't ask for any password.
I'm using my Extreme in Bridged Mode, connected via the Gigabit Ethernet port to my ISPs router. What's weird is that in my ISP router's control panel it shows up under one name and IP address but it's not the IP address that it actually uses, which doesn't show up at all in the router's IP table.