I’m surprised that this worked prior. ’Guest’ networks typically do not allow clients to access anything on the local network. Guests are only allowed to access the public network. SONOS is on the local network.

Has there been a recent router update that may have changed the configuration of the guest segment?

In the case of home and office networks, only one is typically in range and there is no possibility of confusing them or being a guest.

I'm just using 'guest' network as the name as that's the router terminology. It's still a password protected private 2.4 GHz network. I don't need to access the NAS on my usual network as is only used for streaming. 

I switch WiFi networks manually on my device when required (usual network is the default) and could connect to either Sonos network without any issues.

There has been no router update on the logs. I can connect to the two Sonos networks still, but only using two different devices. As soon as I try to switch Sonos networks then no success in either direction. 

I am logged in under different Sonos accounts as I needed to do this to set up each Sonos network with seperate Alexa / Spotify accounts, but again, this never caused me issues previously when switching between the two. 

I am struggling to understand why you have gone down the two network route.  Hardly anyone does this as it is rarely optimal or necessary.  It is possible to set up multiple Spotify accounts on a single Sonos system.  And do you need more than one Alexa? 

If you were to put the second router into ‘access point’ mode, and made the SSID and password the same as the main router, put all speakers into one system, and added all the Spotify accounts, then the whole thing could function as one.  Maybe that isn’t what you want?

If you just want to get your controller capable of connecting to the two different systems on different networks then try the following:

1. Power off the second router or all the Sonos speakers that connect to ii.
2. Connect your phone to router 1.  In the Sonos app go to App Preferences and Reset the app.  
3. Choose ‘connect to existing system’
4. Power on the second network and power off all speakers on network 1.
5. Connect your phone to the second network
6. Hopefully it will offer you the chance to connect to existing system, which this time will be the other system.  If so, please do so.
7. Power back on the other speakers

If that works you should flip system as you flip networks.

As I say, i am struggling to understand why you are in this place at all, but if it works for you, fine.

Thanks John - the reason I’ve had to go down the two network route is (and please correct me if I’m wrong) voice control only works with one Spotify stream, therefore if both my kids ask to play something it will cut the other off, even if they have two seperate accounts?  They only use Alexa voice commands to control their systems and do not have mobile devices.

I’d be happy to be wrong on that point and have a single network as obviously that comes with the advantages of multi-room play which I don’t have at the moment with my daughter’s device?!

I have managed to resolve this now using John B’s steps above.

Would still be interested to know whether you can have two Spotify streams initiated from voice control if on one network as, if so, that would be a lot better. 

Hi. You may be right about the Alexa. What I believe to be the case is:

With a single Spotify account you can play multiple Spotify streams using the Sonos app. But only one stream using Alexa. 

With multiple Spotify accounts you can play multiple Spotify streams to Sonos using the Sonos app.  With a single Alexa app.... I don't know.  It is quite possible that you are correct and it's not possible.