One very quick and simple way is to perhaps reset the S2 App in ‘Settings/App Preferences’ and then on reopening, ask it to connect to the existing S2 system on the same network signal (do not create a new system) - if it doesn’t find it, then try restarting the S2 players and make sure you have not setup the system on a different access point, or guest network.

Additionally, one of the keys to resetting the router is it should be done while the Sonos devices on question are unplugged.

All fixed thanks! Resetting the app worked , cheers.