@Aeron First of all, if you use any router, only connect one single deco unit directly into the router. all remaining router Ethernet ports are tabu if you use the Deco as Mesh router (not in access point mode), unless you are okay with what is directly wired to the router and is not the Deco unit has a different IP subnet.

And yes you need to turn off the Archer C7 WiFi. The Deco units I advise to use as your Mesh Router that is then taking care of IP address leases and the WiFi. Although, fom what I can tell in your setup, the Archer C7 is not needed at all, so you can leave it out of the equation completely, having a separate modem connected to the C7 router and then use the M4 Mesh router, this is technically speaking absolutely not needed. You are fine with just the Virgin modem with turned off WiFi and the Deco M4 Mesh WiFi router.

So my suggestion is to connect one M4 directly to the Virgin modem as described in the instructions and only work with the Deco M4 Mesh router, in case you need more, they can be mixed, meaning you can buy any Deco unit and integrate it into your mesh network.

Try to set up the M4 as your mesh router that is providing you with WiFi and then set them farer apart until you do not get a strong WiFi coverage signal when clicking on the Internet Symbol and then on the individual Deco units, they will show you the connection speed. If you see one orange bar, then you could try to move them closer together, there are 3 bar indicators for the WLAN strength.