Try removing the One SLs as surrounds in the Sonos app. Then unplug the Arc and One SLs from power for a couple of minutes and reboot your router and phone. Swap the One SLs (left speaker is now the right speaker, right speaker is now the left). Then plug everything back in and add the One SLs as surrounds again. Also make sure WiFi is enabled on all speakers in the Sonos app.

Make sure your wifi is broadcasting on channel 1, 6 or 11. 
If the system works for a while after rebooting, this suggests ip conflicts so do it again to clear ip addresses, then assign fixed ip addresses to your devices. Your router manual should tell you how, or post the details and someone on here may be able to help you with step-by-step instructions. 
When you restart the system, be sure to wait till each item is fully restarted before powering on the next item. With router, be sure it’s powered off for a minute or so, and be sure wifi is fully running before powering up your Arc. Start the Arc before sub or surrounds. 

Thanks for the quick response!

I’ve read some quick instructions on how to assign static ip addresses. The following screenshot is from my router’s interface. My question is do I have to leave DHCP on, manually add devices from the link at the bottom but make sure I leave the devices with static ip outside the 2-254 range? for ex. change the DHCP range to 10-254 and add static ips from 2 to 9?

Thanks again for your help!

You have to leave DHCP on or no addresses will be assigned to devices on your network and things will not work as expected.

You’d use the “manually add devices” option.

What ranges you select are up to you, within the allowed 2 to 254 range.

I’d set my DHCP range to 10 to 200 and use the first 8 (0 and 1 aren’t available) for things like printers that are handy to be able to access by address. The majority of my stuff then gets addresses in the 10-200 range. I put my IOT stuff and Sonos up above 200 so it is easy to identify if I need to troubleshoot something.

So I removed Sonos ones from surround mode, unplug from power, same with arc and reboot my router. I successfully add the right Sonos one but I cant connect the left Sonos one. The app shows the right surround speaker ‘’not connected’’ and the left is a single speaker but cannot be added to the system as surround.

After I restarted the app it shows both Sonos ones as seperate speakers no matter if I already set the right as surround.

Just to give an update to my previous post, after a couple of unssuccesful attempts the system has now been connected and working as axpected. I’m not sure what causes all these connectivity issues between the speakers and the app. 

Whats the next step in order to reserve static ips for the speakers? I will reseve 4 ips one for each speaker and then I will have to reboot all the devices plus the router so that the rooter reserves the new ips?

Thanks!

Use your router’s DHCP settings page to reserve the IP addresses, it is usually a simple process and that page usually has a help option or there are details in the router manual.

It is best to reserve, power down AL Sonos, reboot router, power up Sonos as this reduces the chance of anything using stale/cached addresses.

Reserved IPs for all speakers, reboot speakers and router but still connectivity issues and missing surround speakers in the app. In the router’s settings I can see the new IPs but Sonos One speakers are displayed grayed out like they’re not connected.

If you’re still having connectivity issues after assigning reserved IP addressing your router, that’s a pretty strong indication of wifi interference , either from within your network, within your home (non network devices), or even external to your home. 

However, I’m concerned about your ‘grayed out’ issue. Are you certain you’ve power cycled everything after the process? Not knowing which router you’re using is a small hamper, I wonder if it uses the ‘greyed out’ to indicate proxies IP addresses, such as a bonded Sub or surround speakers would use. Never heard of that, but didn’t want to exclude the possibility. 

It might be fastest if you were to call Sonos Support directly to discuss it. None of us community members have the ability to read that diagnostic you submitted, and Sonos won’t look at it until you call in. 

I never had interference issues before (about 2 years) and I have a lot of devices that are connected through Ethernet to my router. My router is speedport plus which is common in European internet providers.  
 

Earlier today I unplugged all the devices plus all Ethernet ports from router and a switch. I connected only the sonos arc straight to the router. Powered on all the speakers and the app couldn't connect to my system at all. 
 

After a few hours everything was fixed and the app was displaying the surround setup and everything worked until now that again the sonos one LS goes missing again. Generally the system is up and running but these connectivity issues are very often.

This is my routers page with the devices of my network. You can see ZonosZP is grayed out.

What does Sonos support say?

Well after chatting with sonos support, the connectivity issue has to do with my Wi-Fi  extender which uses the same Wi-Fi as the router. I moved the extender to another room and the system is now up and running. The app and speaker were connecting to the extenders Wi-Fi causing all this connection problems. As I’ve been told from the support team sonos can’t work with extenders. They actually told me to change the extenders Wi-Fi name so that the app and the speakers doesn't need to connect to the extenders Wi-Fi. Seems that the system is stable until now. 
 

thank you all for you answers!