@woozle thank you for the clarification on the SSID and Hidden networks. I think you are right. It is not an hardware issue. The Routers work fine when setup independently. The software Buggs itself...

@Joseph-TP Sorry for the late reply, I was testing your solution. I have swaped the routers as you asked. The devices still connects to the further Main router rather than the closer satellite router...

I recently bought two TP-Link Archer C6 routers and set them up in a mesh network. Initially everything worked perfectly, but after a few days people on the WiFi started complaining about random...