Two networks - one Controller ?

Not related to your Sonos issue but have you considered switching to a router that supports two ISPs/uplinks and steers traffic to them as needed?

Access points are 30 meters apart - one adsl, one cable .... what router would fit that?

If you connect to the Beam, then power the beam off, can you connect to the other system without resetting? Are any components wired to a router? I assume you are switching networks on the computer?

Are these routers completely separate or connected in some way? Are they both acting as DHCP servers, i.e. handing out IP addresses?

If I switch to network 1 (Sonos Play 1) then Sonos displays a message 'We found Sonos on the router 1 wireless network' . The only choices then from the menu are Mange-Change Language and Help-Reset Controler.

Bothe routers are completely separate (different channels, etc.). Switching networks on one PC. Shutting off BEAM makes no difference.

Access points are 30 meters apart - one adsl, one cable .... what router would fit that?

Probably quite a few, the one I'm most familiar with is pfSense that can run on a custom platform or on an old PC with a couple extra network cards added. Just set the ISP routers to Bridge mode and you'd have one network at the router but could add multiple subnets if you wanted to. They have a forum dedicated to just this:

https://forum.netgate.com/category/21/routing-and-multi-wan

Also all Ubiquity EdgeMax routers support Dual WAN, Edgerouter X, maybe some of their other gear too.

So when one ISP goes down do you lose the devices on that network? I think @Stanley_4 was on right lines. What you need is two internet connections and one LAN and DHCP server. Maybe both ISP routers in modem mode and a third router to provide LAN/DHCP. The issue must be related to your rather unusual setup.

Castalla ... there is an easy solution to this.

You have two routers that are entirely separate in your home...

Router 1 has a Sonos One connected to it.
Router 2 has your new Sonos Beam connected to it.

Your PC simply needs to switch its network connection.

Connect your PC over WiFi to Router 1 and open the Windows Sonos Controller App .. it will connect and see your Sonos One. If it doesn’t then reset the controller and choose to connect to existing Sonos System or Household.

Close the Sonos App.

Now switch your computers WiFi connection to Router 2 and open the Windows Sonos Controller App .. it will connect and see your Sonos Beam. If it doesn’t then reset the controller and choose to connect to existing Sonos System or Household.

To now switch between your two sonos devices you must switch your PC's WiFi connection... then open the Sonos controller.

To get around these problems and to just use one router/controller for all your devices, then the only way to do that is to follow the detailed instructions that I gave to you in your earlier thread here:

https://en.community.sonos.com/what-to-get-228989/shambolic-frustrating-experience-attempting-to-purchase-from-sonos-shop-in-europe-6811600

@Stanley_4. Our posts crossed, but I think you know more about the options here than I do.

@Stanley_4

Thanks but beyond my pay-grade!