Diagnosis
The eero is stuck in its boot/startup sequence and hasn't established a proper connection to the network or the app. This is common after a fresh power-on or reset.
How to Fix It
Power cycle the problematic eero. Unplug its power adapter from the outlet for a full 30 seconds, then plug it back in. Wait 5-10 minutes for it to fully boot and try to connect in the app again. Do not press any buttons during this time.
If That Didn't Work
Perform a soft reset. Hold the reset button on the bottom of the eero for 7 seconds until the LED flashes yellow, then release. This resets the node's network settings without deleting its data. You'll have to set it up as a new node in the app.
Heads Up
A solid white light can also mean it's working normally but the app is having issues. Force-quit and restart your phone's eero app before assuming the hardware is faulty.