@Jonchill 

It is possible that some sort of WiFi signal interference had been introduced, such as new microwave, fridge or cordless phone. Also that interference might have been coming from your neighbours. 

The only solutions are to move that Satellite Deco closer to Main Deco or use Connection Preference feature to configure that Satellite Deco to connect to other Deco with stronger WiFi signal between them. Connection Preference: Customize Your Preferred Signal Source

You can also check for unlikely case that Satellite Deco got hardware issue. Swap it with another Satellite Deco and if that new Satellite Deco also shows weak signal it that location, what you have is unlikely Deco hardware issue.