DIAGNOSIS The node is stuck in its boot sequence or is failing to find/join the existing mesh network. It's in limbo.
FIX First, move this blinking node right next to the main node, plug it into power, and wait 5 minutes. If it turns blue, the original location was just out of reliable range. If it still blinks white, factory reset it. With it powered on, press and hold the reset button for 10 seconds until the light flashes red, then release. Wait for it to reboot (light will pulse white, then solid white). Then use the app's "Add a Node" to adopt it fresh.
IF THAT DIDN'T WORK There's a slim chance the firmware is corrupted. You'll need to reset the main node as well to force a fresh mesh creation. Reset the main node (10-second button hold), set up your network again from scratch in the app, then add the problematic node.
HEADS UP A perpetually blinking white light almost never means dead hardware. It's almost always a software/connection state that needs a hard reset to clear.