DIAGNOSIS The solid red LED means the mesh node has lost its connection to the main router unit. It's in a failed or isolated state, so your devices are dropping off.
FIX You need to re-establish the backhaul link. First, power cycle the problematic node: unplug it for 30 seconds, then plug it back in. Give it 3-4 minutes to try and reconnect. If the light stays red, move the node much closer to the main router, ideally in the same room, and power cycle it again. Once it connects (white light), you can move it back to its original spot.
IF THAT DIDN'T WORK The node might need a hard reset. Find the reset button on the back, use a paperclip to press and hold it for about 5 seconds until the LED starts flashing amber. Release it. This resets the node to factory settings, and you'll need to add it back to your mesh network through the ASUS Router app.
HEADS UP If this keeps happening from a specific location, the wireless backhaul signal might be too weak due to walls or interference. You might need to adjust the node's placement or consider using an Ethernet cable to connect the nodes for a wired backhaul, which is far more stable.