My whole system is gone again...

@Myrdhin,
Perhaps it’s not your Android mobile and that you are looking in the wrong place to solve your issue 🤔…

You mention a WiFi access point, is that wired back to your main router, or is it a wireless WiFi extender? If it’s the latter, it might be that causing you the issue, as it maybe an unsupported type of network… see below list of unsupported networks:

• Wireless internet connections such as satellite, mobile hotspots, or LTE routers
• Guest networks or networks that use a portal login page
• Networks using wireless range extenders2
• Ethernet over Power (EOP) devices
• WPA/WPA2 Enterprise

Above list taken from the Sonos System Requirements HERE

I understand that the Sonos Controllers use SSDP multicasts (the UPnP discovery protocol) over UDP to discover the Sonos players. 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 are that they be set to the same WiFi channel. This is for optimum performance particularly in groups. It appears some AP’s will work okay with Sonos, but others may not, particularly if the AP is perhaps auto-selecting their own WiFi channels and to solve any issues arising, many here (myself included) often suggest using SonosNet for Sonos device discovery/communication instead. 
 

Maybe just see if connecting all to your central router WiFi only solves your issue, by temporarily setting aside the WiFi access point …and if that works, then perhaps blacklist the players/controller device to not use the AP and see if that solves your issue.

Thanks but wifi is well configurated 😄. I think SONOS should read all last bad comments in Google store about their ”app”

Once you insist it is X and not Y, you severely limit yourself from finding the correct answer to your problem.  Now, do you wish to fix your system, or simply whine about the app? 

I have just solved half the problem: my beam is recognized again.  The "super" and so "wonderful" SONOS app has just offered to update the firmware of the beam!  I confirm: the problem comes from SONOS and its app that it would be better to entrust to real developers

Beam firmware updated, had to reset my two one SL to link them again with the beam. All work... until the next SONOS app or firmwares update.

And there seems little doubt, in my humble opinion, that you will most probably encounter the exact same device ‘discovery’ issue again, going forward, because so far it seems as though you haven’t actually (yet) resolved the root cause. I guess only time will tell if that is indeed the case…

I have been professionally "dissecting" wifi for many years.  My network is correctly configured, I have no doubt about it. I'm working with all these protocols everydays.

What I'm also sure of, and I'm far from alone, is that SONOS had better focus on what it does best, which is hardware and sound, and  leave software development as well as third-party services (i.e. assistants, radio, etc) to those who already have much better experiences.

The app is just a remote to your speakers. If the app does not find the speakers, their probably something wrong with the connection of your system to your wifi. The fact other devices do not have any problems does not prove your wifi is problem free for a multiroom audio system.

The app is not only a remote, it's your SONOS "dashboard". Without it, you can't use your system.

If the app has some bugs and report to not see your speaker (perhaps because there is new app and firmware updates to support new voices commandes and resolve somes bugs?), the app guides you to a search for new devices, and/or a restart.  And if all that doesn't work, go to the doc which recommends a reset...