I don't have anything connected to the pc apart from the monitor, however I discovered on the app settings, there is an option to have the server control the app or the app control the server. When I changed that to the second option I was able to select 'Denon CEOL' and it stayed on that option rather than reverting straight back to 'Player'.
I only use the PC and JRiver to add music or create playlists( mainly because it's in another room in the house), I don't use it to control the playback of music, that is all done through the jremote app. I'm hoping that this will now sort me out now, the only issue I have is how long it takes for the app to show the Denon. I will let you know
Thanks
Paul