Hi,

With C7 works as a router but with wifi disabled, the devices can still get internet from the C7 and the advanced feature like parental control, USB sharing will still work. while if the Deco M4 connects to C7 by ethernet cable and works as a main router, there will be 2 routers/DHCP servers, the devices connected to the C7 or M4 won't be able to access each other. So called the double NAT issue. 

While with main deco->slave deco, the devices connect to the Deco will be in the same network, and they can access each other, only one DHCP server existed.

For the C7, you can connect it to the Deco and configure it as an access point to extend the wifi range. Below is the instruction: 

https://www.tp-link.com/support/download/archer-c7/#video

May it help and have a nice day. 

@gonzalo_s 

Hi, C7 would have its own wireless network which is not the same as Deco mesh system;

Thank you.