Diagnosis
A solid red LED means that node has lost its connection to the main router node. This is almost always a communication issue between the nodes, not a dead unit.
How to Fix It
First, power cycle everything. 1) Unplug both the router node and the red-lit node. 2) Plug the router node back in and wait for it to boot up (white light). 3) Now plug the red node back in and place it much closer to the router, ideally in the same room. 4) Press the WPS button on the router node, then within two minutes, press the WPS button on the red node. They should re-pair (lights will flash, then go white). Once paired, you can move the node back.
If That Didn't Work
The node might have been factory reset. Use a pin to press the hidden reset button on the red node for about 5 seconds until it flashes. Then set it up again as a new node in the Asus Router app.
Heads Up
Avoid putting the nodes in a direct line through too many thick walls or large metal appliances. The 5GHz backhaul band they use for talking to each other needs a decently clear path.