All SONOS players and controllers should be on the same subnet. In my own system there is only one router, but there are two WiFi SSID's. The SONOS players are free to use either. As I'm testing things one SSID will go down and any SONOS units that were using that SSID will move to the other. It's so transparent that I no longer keep track of that activity.
I also took a player to work and added the work SSID. When I brought the unit home, it added the work SSID to the other units at home. Now I can simply walk into work with any player from home. Up to 16 players are supported in a multi-SSID scheme.
Note that at work the player(s) from home use my home controller, the home player(s) are not added to the work system. This is fortunate because the home and work systems use different music services and SONOS playlists.