DIAGNOSIS: Your computer isn't on the same local network as the router, often because you're connected to a guest network, a different router, or your PC is using a weird IP configuration.
FIX: First, make sure you're connected to the XR1000's Wi-Fi or wired into one of its LAN ports. Then, try using the router's default IP address directly: type 192.168.1.1 into your browser's address bar. If that works, you're in. If not, on a Windows PC, open Command Prompt and type 'ipconfig'. Look for the 'Default Gateway' address—that's your router's IP. Use that number instead.
IF THAT DIDN'T WORK: The router might be stuck. Do a 30-second power cycle: unplug it, wait 30 seconds, plug it back in. Wait for the white light to settle. Then try 192.168.1.1 again. Also, temporarily disable any VPN software on your computer, as that can block local access.
HEADS UP: If you changed the router's local IP address in the past (like to 192.168.0.1), you'll need to use that custom address. A factory reset is the last resort if you've forgotten it.