I recently had days of my devices disappearing from the app, only to reappear, my radio music stream dropping out , and when trying to reconfigure it failing to find server and new devices etc ..tried all the suggested fixes to no avail. Having a long look at things as aeverething was originally working fine and this only started to occur out the blue with no changes to my sonos or my network i had to look down teh lines of wifi interference, but not seeing anything with my other network devices. But in teh process i founf=d teh the follwoing. My router wifi network is a a BT smart hub, which select the wifi channel best for my enviroment autoselect a channel that least interferes with other neighbouring networks, in my case when i checked it was channel 11. BUt when i check teh app on my iphone the SOnos was set to channel 6 ( settings, systems, network , Sonos channel = 6).. I changed this to channel 11, and all my instability , access to server to update, disappearing devices have been resoved.. my guess is my router was original on channel 6 but a new nearby network appeared and bumped mne to channel 11 ( as my router auto detects), but the app didn’t change but remained on channel 6. ( sorry if this is too technical for some). So problem resolved, So try checking  sonsonet channel in app is same as router wifi channel .. hope is helps others, i was lucky i knew my network was originally stable and something had changed . Originally went down the lines of thinking it was a software update ( my devices were a mix of S1 play1 products updated to S2 firmware, and S2 products running ofn the S2 app), that left only other thing that can change was the network.