Diagnosis
The router has failed its boot-up sequence or can't establish a connection to your modem. This is almost always a physical connection or upstream internet issue.
How to Fix It
Check the cable between your modem and the Nest router's WAN port. Unplug both the modem and the Nest router, wait 30 seconds, plug the modem back in, wait 2 full minutes for it to fully boot, then plug the Nest router back in.
If That Didn't Work
The router might have a corrupted firmware image. Perform a factory reset: press and hold the factory reset button (on the bottom) for about 10 seconds until the light pulses yellow, then release. Set it up as new in the Google Home app.
Heads Up
If the red light persists after a factory reset and you're positive your modem's internet is working (test with another device), the router's hardware is likely faulty and needs replacement.