@coolspot18 

Please show us the current network diagram, how do you connect these devices? From your description, it seems like :

ISP router --Switch--Main Deco )))(((slave Deco

                              --3x Deco M4 

What is the signal strength you can get from the Main Deco's wifi with your smart phone or computer when you next to the slave Deco? 

With locating it further than 10ft, can the slave Deco pick up the main Deco's wifi? What is the led status of the slave Deco M5 at that time? 

Try to exchange their locations and verify whethe it is the Deco itself or the network environment which affects the performance. 

For the ethernet backhaul and wireless backhaul, they won't disable each other, while the ethernet backhaul has higher priority than the wireless backhaul. The reason why the slave Deco does not connect to the Deco M4 which has connected to the switch by wired cable is hard to say, Next to the slave Deco, you can compare the signal strength from the main Deco M5 and the slave Deco M4 separately. 

Good day.