Diagnosis
A solid red light is a failure indicator. It means the point cannot establish a connection to the main router, either because it's too far away, there's too much interference, or the unit itself has a hardware problem.
How to Fix It
First, rule out placement. 1) Unplug the point with the red light. 2) Bring it into the same room as the main router, plug it into an outlet there, and wait. It should go through a boot cycle (pulsing white, then pulsing blue while connecting) and settle on a solid white light. 3) If it connects fine in the same room, the original location was the problem—too far or with too many walls/floors in between. Move it closer.
If That Didn't Work
If it still shows red even right next to the router, factory reset the point. Press and hold its reset button until the light pulses white (about 10 seconds). Then use the Google Home app to add it as a new point, following the on-screen instructions.
Heads Up
These points need a decent wireless signal from the router to work. They can't magic a signal through three brick walls. The rule of thumb is to place them within clear line-of-sight of the router or another point, or at most one solid wall away, for a stable mesh backhaul connection.