This might be a router settings issue, what make is it?

Make sure that multicast traffic is not being blocked in some way. Sonos players and controllers must share the same (broadcast) subnet. Controllers use SSDP multicasts (uPnP discovery protocol) to discover the Sonos players. I understand that some network devices may block this type of communication between wired & wireless connected devices, check your device manual or perhaps check online/support desk.

Note also that if the Sonos is connecting to multiple access points, the recommendation is that they also be set to the same 2.4ghz channel with 802.11b/g modes enabled. This is for better performance in groups, but this alone is not likely the cause of your current issue, but worth doing all the same.