This is eerie, I have the same number and types of speakers and very similar issues, I have a sky router. 
I have spent two years trying to resolve this issue, I’ve bought boosters, moved the router, the speakers, wired used Sonos net and WiFi. I’ve spent literally days on the help line, I’ve spoken to the top tier of technical support and guess what? It still don’t work. 
I have an email from a guy at technical help that states “he’s with me till this is resolved” (happy to share) but every time I email him I get a generic response taking me through the basic steps, router reboot etc.

the last email he sent me (before the generic BS he told me my expectations were too high as I don’t live in a “pristine environment”

go figure!

good luck my friend 

Did you disable Airtime Fairness.

Have you tried it locked to 20 instead of 20/40?

Have you tried assigning static/reserved IP addresses, powering down all Sonos and rebooting the router before powering them back up?

This is the problem, I have no idea what you refer to, I’m not technically minded, I bought Sonos from a large department store, I feel duped as I was told it was plug and play, I believed the Sonos marketing which told me that they had a solution that allowed me to have speakers in my cellar and in my attic, shame on me for believing their nonsense 

thanks for trying to assist 

Hi Stanley _4,

I didn't know about the airtime fairness functionality, disabling it seems to improve connection to Spotify a bit but devices keep disconnecting.

I tried all bands (20Hz, 40Hz and 20Hz/40Hz) and all channels without success.

Hi Stanley_4,

Disabling Airtime Fairness worked on Spotify connexion for a day and then it failed again. Now I still have both Spotify connexion issues and devices drops even on local music.

Actually all actions I do make the system stable for few hours or maybe few days and then problems come back. There must be a stable solution because sometimes it works as a charm.

Because you have your Beam wired (hopefully direct to your router and not via a switch etc.) your devices are unlikely to be running on your WiFi network, the devices are most probably all running on SonosNet.

Having a main Home Theatre device wired as the root bridge device is ‘okay’,  but perhaps not as ideal as say having a standalone device like a Sonos Boost for example, or a modern standalone speaker, as the HT processor and memory are also often used for TV surround sound, which in itself can be quite intensive. So there are some things you can also do with your setup aswell as disabling Airtime Fairness. Here would be my suggestions…

• Switch the 2.4ghz band away from the ‘overlapping’ channel 3 and place it on channel 1, 6 or 11… these are non-overlapping WiFi channels and set the channel width to 20 MHz.
• Next goto the Sonos App “settings/system/network’ and set the SonosNet channel so it is at least 5 channels away from your chosen router channel.
• Also whilst in the Sonos ‘Network’ settings, goto ‘Wireless Setup’ and reset/remove the local WiFi credentials as these are not required when running the system on SonosNet and it will stop the speakers potentially jumping between a SonosNet signal and the WiFi signal.
• If you can also wire a standalone speaker to the router instead of, or aswell as the HT device, then I would perhaps do that too, but maybe unplug the HT device initially, so that the other standalone wired speaker takes on the role as the ‘root bridge’ for the system.

Hope the above assists.

Thank you Ken_Griffiths for your advices, removing the router WiFi from the Sonos app to force Sonos mesh to do all the job seemed to improve the situation a bit these past few days, I still have some drops though. And actually I also did that a few weeks ago but with no good results, why did it work better this time?
That's a mystery.

I don't really see the issue with having the Beam wired instead of a standalone speaker as I obviously don't listen to music when I watch a movie. I also don't understand why sometimes it's the Beam that drops while all the other speakers are working fine, can it depend on which speaker is on top of the playing devices list in the room?

I think of getting a boost to strengthen the Sonos mesh, does anyone think it can improve the situation? I'm a bit frustrated to have to pay extra for a feature that should work fine by its own and not even knowing for sure that it will solve it once and for all.

Did you try a ‘different’ fixed (non-overlapping) 2.4Ghz router and SonosNet channel too? .. just ensure whichever two channels you select, that you keep them at least 5 channels apart from one another and set the 2.4Ghz channel-width to 20MHz if you are able to change that.

Yes I did : 20MHz on router with channel 1 and Sonos net on channel 11.

Hi @Matmat 

Are you still having this problem? Before spending any money on additional equipment, please check the instructions below first. If they don’t help, please make sure no Sonos devices are within 1m of anything that transmits WiFi.

If you are then still having problems, I highly recommend you get in touch with our technical support team so they can get diagnostics from your system - it may just be electromagnetic interference, presumably near your Beam. 

1. Download the Asus app on your mobile device (required to perform these steps).
   
2. Confirm the mobile device is connected to WiFi and launch the Asus router app.
3. Tap Devices.
4. Scroll to the blocked Sonos device and tap on it to open a new menu.
5. Disable Block Internet Access.
   
   Blocked Android device example:
   
   
    
6. Reboot your router and Sonos units and test.