Diagnosis
The wireless backhaul connection between that satellite and the main gateway is unstable. Signal is being lost.
How to Fix It
Move the problematic satellite. It needs a stronger signal from the gateway or another satellite. The ideal placement is halfway, with clear line of sight. Avoid placing it inside cabinets, behind large metal appliances, or in corners. After moving it, unplug it for 10 seconds and plug it back in. The light will pulse blue then go solid white when it reconnects.
If That Didn't Work
The satellite may be too far through too many walls. Run an ethernet cable from the gateway eero to this satellite's WAN port. This creates a wired backhaul, which is always more stable. The eero will automatically detect the cable.
Heads Up
A red light can also mean hardware failure. If it goes red immediately on boot in a known-good location and with a wired connection, contact eero support. The unit is likely defective.