It sounds a bit like there is something on the phone blocking the ‘discovery’ of your Sonos devices on the local network, what type of phone make/model/OS version is it? Do you have any security software running on the phone, such as a VPN service, firewall or antivirus etc?

Any MAC address ‘spoofing’ features running on within the network connection properties, such as ‘Private WiFi Address’ (usually enabled by default on Apple iOS mobiles), or maybe ‘WiFi calling’ is getting in the way of things? I understand in iOS that it can be useful to switch off ‘mobile data’ access to  the  Sonos App too - so perhaps give that a try.

Also ensure QoS and/or Airtime Fairness is not running on the local router …and (if applicable) that IGMP Snooping and Multicasting/Broadcasting is enabled across both the LAN and Wireless LAN segments (most routers enable this be default, but not all, so check your router user-manual). 

If using managed switches on the local network, then you may also find these links helpful…

https://support.sonos.com/en-us/article/configure-stp-settings-to-work-with-sonos

There might also be an issue of device discovery arising from Wireless interference, in which case also see this Sonos Support article…

https://support.sonos.com/en-us/article/reduce-wireless-interference

Also see..

Wireless Interference Video:

Wireless Interference and Sonos

Hope the above information helps you to resolve the matter. If the problems persist then it maybe best to speak to Sonos Support Staff, together with a diagnostic report, to see if they can perhaps  assist you to resolve the local ‘device discovery’ issue.

Thanks Ken, that all sounds incredibly complicated for something that used to pride itself on “just working”!  I have switched the 5G network off on my router as I suspected some of our iPhones were using that while the sonos is all on the 2.4G network.  I’ve also removed the network from ‘trusted networks’ and re-added it.  I’ll continue to monitor it and see how it goes.

I was thinking of buying a Boost, because none of my components are near an Ethernet tail any more (I used to have an ARC wired into my router but I sold it and now Sonos is on WiFi mode rather than Sonosnet.  I have been a Sonos user since the early days and have always had one component wired in until I sold the Arc.

Slightly dismayed to see the Boost is no longer sold though!  How is one meant to start a sonosnet network now if there isn’t a component sited near ones router?

Also dismayed to see Sonos Support, which used to be absolutely excellent, is now Mon-Fri office hours only.

Just to add you can cable any speaker/device to the local router to cause all products to use wireless SonosNet except for…

• Roam/Roam SL
• Move/Move2
• Era 100/Era 300

The above do not use wireless SonosNet. 
Rather than buy a Boost I would personally try a different router instead - that’s if you eliminate it being the mobile devices & any switches and believe the router is having some difficulty with resolving the multicast packets across the different network segments. I’ve certainly seen that issue reported here in the community in the past (Some BT routers etc. in the U.K. just as an example).