A few of things to check. 

First, all these devices are part of the same Sonos software system? Right now, can you see the Beam’s room and the PLAY:1s room in the controller?

Second, you haven’t by any chance turned off the WiFi on the Beam, have you? Might be worth toggling it off and on, just to make sure. 

Third, it might be worth rebooting your router, just to ensure its DHCP table is refreshed. 

Some people have reported that they have to set up the surrounds as a separate room first, before then adding them as surrounds, but I’m not sure that is relevant in this case, but thought I’d mention it.

At the end of the day, if none of this helps, you may want to  submit a system diagnostic, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.

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 phone folks, there are more options available beyond just the diagnostic analysis.