My two Sonos Boost no longer show amont my system devices.

I am using Sonos controller 2, have 3 Sonos:5, 4 more Sonos:1. 
Some of them are routinely disconnecting. That’s how I found out my two Bossy were offline. 
I updated the App and the system. Restarted everything (router, each speaker, Boost). Reset each Boost and reinstalled. 
No error message. But they simply won’t show in the list of my devices. 
when I check network for each speaker, it simply says « network not available », although it works (some of them never disconnect, so they are connected to the WiFi).

Is it something with the software, or something I missed to keep using the Boost? It used to work with the older version of the App.

Diagnostic confirmation ********** in case Sonos support is willing to help. No chat available in my country and hotline announces a 40 minutes waiting time…

Moderator edit: Diagnostic number recorded and removed (identifying information)