I’d be tempted to try a factory reset on it, to force a reload of the OS…. I’d also try wiring the Arc directly to the router with an Ethernet cable, at least temporarily while setting it up. 

If neither of those fix the issue, maybe posting some additional details would help, particularly about your router and network. 

Hi @Sam1970,
Perhaps start by ensuring your local network meets the Sonos system requirements shown here:

https://support.sonos.com/en-us/article/sonos-system-requirements

These are the App requirements too:

https://support.sonos.com/en-us/article/sonos-app-requirements

Just to add that myself and members of my family & friends, all have the same Soundbar and the setup in every case was a ‘breeze’, so if it’s not a Sonos hardware/software fault, then I would look closer at the various requirements above.

There is also a known network hardware incompatibility list here too (just in case): 

https://support.sonos.com/en-us/article/incompatible-network-hardware

Hope you get it sorted.

Also see how to factory reset the product (last resort option):

https://support.sonos.com/en-us/article/reset-your-sonos-product

Thanks for the reply. Unfortunately wiring is not an option as along way from router. It is working using TV remote.. Just can't connect to the app 

Do you have another Sonos product that you could easily wire (temporarily) to the router - as that would at least switchover things to use a SonosNet connection instead of your WiFi?

What’s an “audio pass code”?

Note that the Ethernet cable doesn’t have to be permanent, just a temporary thing to get the device kickstarted. 

Just to perhaps assist here a little further - switch off any ‘other’ WiFi access points (if present) and just use the main router only (it’s worth rebooting the router aswell and switching off any ‘guest’ networks, which may just confuse things). Consider using different DNS servers such as Google’s Public DNS 8.8.8.8/8.8.4.4.

Also (if relevant) temporarily connect the mobile controller device to the routers 2.4Ghz band, rather than the 5Ghz band, as that can often assist with Sonos multicast device discovery. Make sure the mobile controller is not running any security software, such as a VPN client or firewall etc.

Thank you very much all. It has been a long night but if finally worked. Not sure what was the solution as I tried many of the suggestions and it worked using a 10 metre ethernet cable...what a faff? Lets hope I will have a better luck connecting the two one SLs (Rears) when they arrive in few days

Thank you so much for guidance.