DIAGNOSIS The node has lost its connection to the main router or the internet gateway. This is the Velop's universal "something's wrong" indicator.
FIX Power cycle the whole system. Unplug the main router node AND the problematic red-light node for 30 seconds. Plug the main node back in, wait for it to get a solid blue light (could take 2-3 minutes), then plug the satellite node back in. Give it another 5 minutes to re-establish the mesh link. The red light should pulse white, then turn solid blue.
IF THAT DIDN'T WORK The node might be too far from its parent node or there's severe wireless interference. Move the red-light node much closer to the main node, ideally in the same room, and plug it in again. If it goes blue there, you've got a range or obstruction issue.
HEADS UP A solid red light on a node that was previously working almost always means a temporary mesh hiccup, not hardware failure. The power cycle fix works about 80% of the time.