How to fix disconnecting system?

We can’t see your diagnostic, since we don’t work for Sonos, sorry.

I’d recommend not doing factory resets, they usually do two things, obfuscate the issue, and delete any potential diagnostics that would help Sonos help you.

If I were to guess, and I think I am guessing, there’s some network issue going on. I’m going to suggest one easy thing, a simple network reset. Unplug all of your Sonos devices from power. Reboot your router. Wait a couple of minutes for the router to come back up, then plug in your Sonos devices to power again. If you can, reboot whichever device is running the controller while the router is off, too. 

If that works, I’d highly recommend looking in to assigning reserved IP addresses for at least the Sonos devices. Instructions on how to do so would be in your router’s manual. 

While you didn’t say how your Sonos is connected to your network, I’d be tempted, before plugging the Sonos devices back in to power, to connect one Sonos device to your router with an ethernet cable. Once everything is up and running again, and you’ve double checked the wifi network info, you could then remove that cable.

There’s always the potential for wifi interference  in any Sonos system, but based on your description, I’d say that it is a low potential right now. And some of the above might remove at least some of that from the equation, which is ideal. The fewer variables in the test case, the better. 

Strangely the system will

work with alexa commands and stream music from my Spotify, so seems the speakers are connected but not the iOS app to control them??

If Alexa works then the players have an internet connection. 

Describe the new router/WiFi arrangement. Is there a WiFi mesh? It could simply be that the local network has been split into two subnets by an additional router, and the phone is on a different subnet from Sonos.

Hi yes that’s the case, there’s a wifi mesh. 
it’s still the case  this morning that the speakers will connect via alexa; the system now shows up on iOS after I reset everything but now it says “unable to connect to speaker” for all of them. 

More info would be handy. What make/model of WiFi mesh? Is the WiFi still enabled on the main router?

Is the Sonos connected to the new mesh WiFi? Kindly confirm.

Thanks. It’s a Nova mesh network, running off a new virgin media hub 5. 
 

having been through a second call with Sonos support to get my wife set up, it’s failing to connect either phone to Sonos - but the speakers themselves are still running off alexa… So i will have to delve into the mesh advice above and sort out a bridge mode!

thanks