The discovery of the Sonos products by the Sonos controller App is a slightly complex area to explore, but briefly… 

SSDP (the UPnP simple service discovery protocol) is used to initially discover the Sonos players and is done via multicasting via UDP to 239.255.255.250:1900 and, for good measure, broadcasting to 255.255.255.255:1900. 

Both the Sonos players and controllers must share the same network subnet. However if the Sonos devices are connecting to multiple access points, the recommendations (from myself) are that they be set to the same WiFi channel and channel-width. This is for optimum performance particularly in groups and for some reason with some (not all) mesh-based WiFi systems, it appears some will work okay with Sonos, but others may not, particularly if the satellites/AP’s are perhaps auto-selecting their own WiFi channels. I’ve seen some people report issues with Google ‘nest’ mesh WiFi, for example, although others will occasionally say it works for their ‘small’ Sonos setup.

What is certainly not supported though by Sonos, are wireless range extenders, as some can mangle device MAC addresses and EoP (powerline) adapters.. so if you are using those type of wireless access points, to extend the wireless network coverage, then that may cause such intermittent device discovery issues by the Sonos Controller App.

Just to add aswell, it can sometimes be Apps/software running on the mobile device that interfere with ‘device discovery’ Examples to perhaps consider are…

• VPN Client 
• Firewall software
• Antivirus software
• WiFi Calling/Dial assist
• Private network address (MAC address spoofing)
• Mobile data enabled for the Sonos App  

Thanks!! Great explanation. I got them all on same WiFi and off the extender. All are working.