Yes it’s clearly a weird one you have here. Agree if HiFi Cast reads the Server Library then the DLNA Server is discoverable on the phone.
Yes I would say this is correct. The Phone is the Control Point. I tried to explain what I understood in this post No music when link Rooms together - #10 by NWT.Stuff
Definitely clutching at straws here. Have you tried uninstalling the 4STREAM APP and reinstalling ?
Sorry not got anything else to suggest !! Good Luck 