DIAGNOSIS This usually means the main router node isn't getting a valid IP address from your modem. It's a handshake issue between the two boxes.
FIX Start with a full power sequence. 1) Turn off your modem. 2) Unplug the main XT8 router node. 3) Wait two full minutes. 4) Plug in and turn on the modem, wait for all its lights to settle (about 1 minute). 5) Now plug the XT8 router node back in. Wait another 2-3 minutes for it to boot and negotiate with the modem. Check the app again.
IF THAT DIDN'T WORK Log into the router's web interface (at 192.168.50.1). Go to the WAN section and check the connection type. For most folks, it should be set to "Automatic IP." If it's set to something else like PPPoE, that's likely the problem unless your ISP specifically requires it.
HEADS UP Some modems "lock" onto the first device they see. The power sequence forces it to forget the old connection and recognize the XT8. If you still have trouble, you might need to call your ISP and ask them to release the MAC address lock on their end.