Related to this issue, but none of the proposed solution helped me with my symfonisk speakers. It helped shortly with a Sonos One SL, but in the end I had to remove it fully from my router and do a full factory reset to get it to work.. 

But I still have the issue with my Symfonisk Speakers, I have two lamps that are connected as stereo, where the volume slider is missing when they are a part of a group. I can also not send music directly to them to play standalone but I’m able to put music on another speaker and then add the symfonisk the that group to get them to play.. 

Edit: It seems that my Symfonisk Højtalere, is in a room called that, but when I go in to change their room name it’s just empty, if I set a value it doesn’t change.