I've recently discovered this application and it's really amazing and useful.
What is working not so good is the media remote control plugin, because it's
not able to automatically detected if you are using a compatible media player
like amarok or vlc (this should be documented somewhere). I've found a
workaround that consists in tinkering with KDE Connect options in
systemsettings: you need to disable and to enable multimedia control receiver,
than on the smartphone app the media player will show and will be working. I'm
guessing if this is a problem on the app side or on the KDE side, what I know
it's that the app is complaining about the KDE counterpart being older.
Valerio