@Adie-H 

I have 4 sonos devices and have used the Deco sucessfully in AP mode and Router mode but your setup has to be suitable.

If you use your Deco's in router mode, you should not have them connected to another router, unless the router is in bridge mode. If the other router doesnt have a bridge mode then you are going to experience NAT probelms double nat and possibly DHCP problems and routing issues. You must make sure the original router wifi is turned off.

The other interesting thing about Sonos is that they can create their own independant wireless network, separate from your main network, which gives them some additional reliability. To ensure they work in this mode you do need to connect one of your sonos devices by ehternet cable to a switch or one of your decos. Then the sonos will create a wireless network directly with the other sonos devices. It shouldnt be necessary though if you dont have two routers in your network.

If you could supply a little more info about how your decos are connected (wired / wireless) and also what your main deco is connected to it might give a few clues as to what is going on.

Oakmeister