Thank you very much for your attempts to help.
Before reset I tried all the methods 
(OFF/ON, search devices, reboot router 
, etc). I read a lot of posts here…
I've had SONOS for a few years now and the more time passes, the more I see the quality drop. I disassembled some (old models) to see what was inside. I isolated systems to listen and analyze the different network frames and protocols used. To be honest, I'm afraid of the new hardware that will soon be. The Roams are far from being of good quality (problems with battery, connection, sound...). The Android application is very far from being ergonomic. I also really like the steps to try to find your system, steps that end with "let's try again later..." rather than sending a report to support with a request for comment, for example.
I solved my problem last night and I confirm: there is a problem during the exchange of frames allowing the discovery of other elements of a system. But when I see the answers of some knowing people, I prefer to avoid making them lose the illusion that SONOS is a bug-free system 
.
Thanks again,
I’m not seeing the issues you have described with disappearing Sonos players etc. The setup I have here, with a WiFi mesh system currently running in bridged AP mode, runs perfectly well, I can’t recall a Sonos product disappearing from the controller App, like you describe, although I have seen others with network issues that have had that issue in the past, for example users with a BT Smart Hub 2 had an issue with multicasting not working correctly between its two different WiFi segments. That matter was later resolved with a router firmware update.
I still personally think you may have a multicast broadcasting issue between your router and the mentioned access point and that with differing channels/channel-widths the packets are being held up and not always arriving at their destination. I would still perhaps decide to take a closer look at the WiFi access point described in your earlier post and also look what security software is running on your controller device that may possibly be holding up the multicast broadcast packets. Otherwise I think your issue will (likely) re-emerge and if it was the Sonos software, then in that case all of us would be having the same issue and clearly that is not the case.
If anything, I still suspect it’s your local network and that it is not as robust as you perhaps believe it is. As mentioned earlier only time will often reveal these things, so let’s see what happens now that you have your setup working again.
Note: It may not resolve your issue entirely, but (if not done already), you may find it helpful to reserve all your Sonos IP addresses in your routers DHCP reservation table and set all your WiFi access points to use the same ‘fixed’ non-overlapping channels and set their 2.4Ghz WiFi band to a channel-width of 20Mhz only - it may not solve things entirely, but may reduce the frequency of you encountering the device-dropout issue.
On your mobile controller device (if in use) switch off MAC spoofing/Private Address, WiFi Calling, VPN Client, or other similar security software etc. temporarily, to see if that may improve discovery too. HTH
