If the router has a wireless access point of any kind and the radio in the router is turned on, you won't see the Sonos unless you're connected to the router's WiFi because the WAP is its own node and while you can ping the Sonos and other devices, it won't necessarily show up or be controllable. I just ran into this with an Arris gateway in a house where radio goes to die and I needed to use multiple Apple wireless devices (they already had them and didn't want to replace) for best coverage. Once I turned off the Arris WiFi and added another Airport Express in that location, the problems ended. I could see two of the Sonos Connects when I was in the area of the Arris, but the Sonos 3 in the kitchen was missing. If I was in the kitchen, I could see the 3 because it was connected wirelessly to the AP in that area.

The system has two Connects in the cabinet where the AV system is located along with the router and a receiver for the other speakers, with one Connect on the AVR and the other is on the receiver. One Connect is hard wired to the gateway and the second is connected to the second ethernet port on the other Connect. One port on the gateway connects to a Netgear Gigabit 8 port switch in the basement, which connects to two Airport Express, and an Extreme, as well as the access point for the U-Verse wireless TV receiver and a Cat6 to the basement AV cabinet, which has another switch for the AVR, AppleTV, Time Capsule, XBox One, U-Verse receiver and a cable to the TV, which isn't used at this time.

Once I turned off the WiFi on the Arris and installed the last AirPort Express, it cured the problems with losing the kitchen Sonos 3/not seeing the Connects in some areas and it also improved the range- I can drive by and pick up the SSID from the street without stopping, in front of both neighboring houses and that's pretty good, considering the heavy masonry exterior walls and metal lath & plaster interior surfaces in all areas.