DIAGNOSIS The router is in a factory reset/recovery state. A solid yellow LED indicates it cannot establish a stable connection to Google's servers or is awaiting setup after a failed update.
FIX Perform a full factory reset. Press and hold the factory reset button on the bottom of the router for 10 seconds. Release after the LED pulses yellow. Wait 5 minutes for the unit to complete its reset cycle. Open the Google Home app, tap Add > Set up device > New device, and follow the setup prompts from scratch.
IF THAT DIDN'T WORK The unit may have corrupted firmware. Use a hardwired Ethernet connection for setup. Connect a laptop directly to the router's LAN port with a cable. During setup in the Google Home app, when prompted to connect to the device's temporary Wi-Fi network, the app will detect the wired connection as a more stable path for provisioning.
HEADS UP After a factory reset, all existing network settings, including your custom network name and password, are erased. You will need to reconfigure your entire mesh system.