DIAGNOSIS That solid red light means the satellite unit has lost its connection to the main Deco router. It's not getting a good signal, so it can't extend your network.
FIX First, let's try the simplest reboot. 1) Unplug the power from the red-light satellite. 2) Wait 30 seconds and plug it back in. Give it a full 3-5 minutes to boot up and try to reconnect. If the light stays red or cycles colors, you need to move it. 3) Physically pick up the satellite and plug it in halfway between the main Deco and the problem area, but still within clear line-of-sight if possible. Avoid corners, inside cabinets, or right next to large appliances.
IF THAT DIDN'T WORK The connection might be too weak. In the Deco app, go to the satellite unit's icon and look for the connection quality. If it's 'Weak' or 'Poor', you have to move it closer. There's no way around the physics of the wireless signal between the units.
HEADS UP Don't factory reset the satellite right away—that's a last resort. A reset means you'll have to add it back to the system through the app's setup process again, which is more work than just relocating it.