That seems odd, certainly. I can’t think of any reason why, once they’ve connected to the Beam, they would then disconnect, and reconnect to your WiFi signal.

Have you tried a reboot of your Beam, followed by a check for updated software in the controller?

Yes, I did reboot my Beam and both Era 100s but still giving me the same issue. I still disconnects automatically from the network whenever I try to create a surround system with the beam.

Interesting. That suggests (but not to 100% knowledge) that your router doesn’t like proxied IP addresses / connections, which is what occurs when you set up a pair of surround speakers. 

I think I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Oh. Simpler potential answer. You have the radio/wifi turned off on your Beam. Or the surrounds. 

Which may need more ‘color’. Try flipping that setting off and back on in your controller, for each device. It’s pretty rare for that setting to be ‘stuck’, but worth a shot.