I suggest that you connect the device with an Ethernet cable and then check for updates.  If the device is updated disconnect and thhen check if the device is visible in the app.

If it's still not working, then please factory reset the device and re-add as a surround.

Thanks for the response but I’ve just tried everything you said. The problem is that (after also the reset) the app see the sonos one like a pair stereo but if you try to push the botton “add surround” in the system with the arc and sub, the app doesn’t recnognize the surround. i don’t undertand why…

Have you turned wi-fi off on the Arc or the One's?

No I didn’t

indeed the one does work and the arc too, but they don’t work togheter 

Strange and it was working before?

Can you try removing the sub and then try adding the surround speakers again, if successful then add the sub.

If that doesn't work, after the Ones have been added as a stereo pair (this will retain your settings) please reset the Arc and then add the sub and surrounds again.

You wouldn’t believe me but i’ve tried everything you said but it doesn’t work. The sonos support moreover doesn’t respond... 

Okay, please retry the adding of the surrounds then when you hit the problem, submit a diagnostic and post the number in this thread.

One more thing to try, please power down all the devices for 30 seconds and retry.

I will try. Thank you very much!!

Try a router restart, with all Sonos speakers unplugged from power. When router is up and running and wifi established, power up the Arc. When it’s fully running, do the surrounds and sub, one at a time. This will force fresh ip addresses to be allocated to the speakers. 

Ok i’ll try and i tell you

It works… yeahh!! Thank you very much!!! 

Great news and well done @nik9669a 

Have a look at my post here to stop this happening again.