Hard to tell from the limited data, especially none concerning the network setup. I’d hazard a guess at either wifi interference or possibly duplicate IP address issues…although it could also be a mesh network / subnet issue. If you’re unwilling to call Sonos Support directly to discuss it, I’d suggest at the least a network refresh, by unplugging all Sonos devices from power, and then rebooting your router. Give the router a couple of minutes to finish its reboot process before plugging back in the Sonos devices. But that only addresses one of those possibilities, and is not a silver bullet that addresses every potential issue. 

Thank you Airgetlam34249 for your reply. I'm not a network expert so don't know about duplicate IP address issues, etc. I have made no special setup than wifi as in other devices. No wifi interferences with any other connected device. Sonos works perfect during a period of time and then the application mysteriously disconnects and only connects back again when router is rebooted. I also did unplugging Sonos devices from power and rebooting router, but issue persist: app disconnects after x days. Dianostics sent but no answer. 

Did you call in? As indicated in the Diagnostics - How do they work? thread, simply submitting a diagnostic won’t generate a response. 

Yes. of course. I contacted them and they told me to send diagnostic.

I have just received answer from them today.

It appears from the diagnostic that the problem has to do with multicast and that it could be solved by enabling Proxy IGMP and IGMP Snooping in the router. I will check if this can solve the issue.