Sorry. I have no additional ideas. You appear to be using the correct tools. Early in the discussion I explained there were known issues with that tutorial that dates from 2014 Herin. Someone else might have suggestions about how to get this working.
Accept what the app can do (you have had some success) or do something different. Perhaps create a chat app on a CloudDB or FirebaseDB . A chat app would provide similar communications functionality for a group of users, but in a very different way.
Regards,
Steve