I recently purchased Deco X20 and had problems with my Sonos. It has a lot to do with which Sonos app you're using. Sonos is having issues with the Sonos S2 app. I am not tech savvy at all so I called Sonos tech support. I wish I could remember everything they had me do but most important they had me connect one of my Sonos devices with Ethernet cord. I know this isn't much help but it may not be the Deco or you but the Sonos app and you need to call Sonos support. They were very patient and helpful. 

@Carl 

Hello, I have a sonos move and a tp-link AX3000 Archer AX55 and was able to successfully set it up with the following config:
 

• I disabled the guest network initially (not sure if this is relevant or not). 
• I also disabled the 5Ghz band on the primary SSID initially
• OneMesh is disabled
• IPv6 is disabled
• I'm using a reserved IP address for my music libary in SONOS via DHCP settings
• Likely not relevant - b.ut I'm using Century Link 1G fiber.  On the IPTV/VLAN tab I'm in USA-CenturyLink mode (internet connection type PPPoE)

After getting Sonos iOS app working with the Sonos Move, I enabled guest network with a different SSID and also the 5Ghz for the primary SSID.

Good luck

  @scottnasello Thank you so much for posting this! Factory resetting the Sonos and disabling the 5Ghz band on the Deco during setup was the winner for me.

  @Carl I had the same issue when I set up my Decco mesh. I plugged two of my 9 Sonos Plays directly into the main Decco and a sattelite. In the Sonos app, under Network, I changed the default SonosNet channel from 11 to 1. All works great now. 

  @StdPoo Thanks - just bought X50 and was dismayed that my Sonos wouldn't work. Just plugging one of them directly to the router instantly solved the problem.

  @Carl I have the TPLink Archer AX90 plus 5 other mesh extenders. Yup 5!!! Thinking the issue why all my Sonos were bouncing in and out was due to insufficient coverage. Then after trying every possible recommendation out there, I discovered the Advance Wireless setting that allowed me to change the 2.4GHz bands from b/g/n/ax to just b/g/n after setting the channel to auto, the band to only use 20MHz didn’t help.

  @WesGardner Worked perfectly! 

Here is a solution. 
 

i had trouble connecting sonos after i made a new network with X20. 
I tryed the AP mode, and later 2.4 network. 
It didnt work for me. 
 

Then i extended the mesh nerwork from the X20, with my old Apple Extreme. 

Then i could setup up sonos 😊

And after the sonos setup i disapled the Apple Extreme, and sonos could Connect to x20 😊

  @Carl This helped me as well, thank you. I couldn't see my Sonos system after hooking up a Deco W7200 mesh network. I unplugged all my speakers and hooked up the Sonos Port to my primary router via ethernet. Once that was connected, I was able to turn on and factory reset each speaker and add them to the network. Once those were connected I could unplug the ethernet cable and move the Port back to its original position. I then gave each of them a reserved IP address through the Deco app. Hope this helps someone!