Note that rear (surround) speakers don’t connect directly to your router, they instead connect to your sound bar, and get proxy IP addresses from your router through that connection.
It sounds like in some manner, you’ve factory reset the Beam, if the system is recognizing it, and asking you to add it to your Sonos system. However, if that’s the case, it should not be recognizing any surround speakers, unless they’re normal Sonos ‘room’ speakers, and not set up as surrounds yet.
Have you tried to call Sonos Support directly to discuss it? It seems like you just have a configuration failure in your network, but there’s not enough detail to tell, the Sonos support folks would have a better opportunity to assist than those of us in this community forum.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.