Something sounds odd. Both your Era 300s, the Beam, and the Sub mini should be in the same Sonos ‘room’, not in separate rooms. What I suspect you need to do is re-add the Era 300s as surrounds for the Beam. Go into the ‘room’ in the Sonos controller for the Beam, and use the ‘add surrounds’ process. The two Era 300s need to be in separate ‘rooms’ before you can do so, they can’t be a stereo pair.
Once that’s done, check the HDMI connection for the Beam, it should be plugged in to the TV’s ARC or eARC port. There’s usually only one, and it is labeled. Once you’ve checked that, it’s worth unplugging both the TV and the Sonos Beam from the wall, then plug them back in.
Alternately, you could call Sonos Support to discuss it.
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
