Do you have a Mesh Wi-Fi network?  If Yes, I suspect that it’s changing channel due to interference.

Have you loaded a Wi-Fi analyser app onto your phone to check for Wi-Fi interference?

Thank you for the reply.

Unfortunately I don’t know what a mesh network is. 

I have a Sky Wifi Router.

My skill’s / knowledge are both very limited.

If you also have a Sky Q box then see the Sky section in the following thread:

Thank you Mr T

I think the Sky Q could be causing the issue. I will try to disable when I get home.

I think I now understand the Mesh question.

Is this the 

MS : 0

On the speaker settings?

If so my Sonos 5 is on MS : 0 But the Arc is on MS : 1

If that makes sense.

Do you actually mean WM:0 and WM:1 ?

Is your Five wired to the router? Did you perhaps disable WiFi on the wired device? That would explain why the Arc is still on WM:1

The Disable WiFi setting actually stops the wired device creating SonosNet for the other Sonos devices to connect through. You can re-enable the WiFi setting in the device’s room settings.

Probably WM 

Neither device is wired. Both on WiFi. I haven't turned of any WiFi as far as I know.

Not really sure what the WM is

WM means Wireless Mode. If you don’t have any Sonos device wired, then you will not see WM:0

When I had both Sky router and Sky Q, I never had a problem with rooms disappearing from the app as I always used a wired setup.

Unfortunately my router is too far to be wired. 

My sky q box is next to my sonos equipment.

One of my two devices said wm : 0

For sure. Maybe this is the problem.

Can you power down both devices for 5 secs and then power them back on.  Please let us know the WM setting for each device afterwards.

Wanted to Say Thank you for your help.

I tried the things you both suggested and it seems to of fixed the problem.

Wanted to Say Thank you for your help.

I tried the things you both suggested and it seems to of fixed the problem.

Thanks for the update. Glad to hear your system is now running successfully. Happy listening!