DIAGNOSIS: The router itself is powered on and happy, but it's not getting a signal from your modem. This is usually a physical connection issue or a problem with the modem's handshake.
FIX: Let's start with the basics. 1) Unplug the power cable from your modem. 2) Unplug the power cable from your XR1000. 3) Wait a full 60 seconds. 4) Plug the modem back in and wait for all its lights to settle into their normal state (this can take a few minutes). 5) Now plug the XR1000 back in. Wait for it to fully boot—you'll see the white light go through a sequence. This order is key.
IF THAT DIDN'T WORK: Check the Ethernet cable between the modem and the router's yellow Internet port. Try a different cable if you have one. Also, log into the router interface (routerlogin.net) and run the Internet Connection Test under the 'Internet Status' menu—it might give you a more specific error.
HEADS UP: If you recently switched ISPs or got a new modem, you might need to release and renew the router's MAC address in the Advanced Settings, as some providers lock the connection to a specific device.