Using the old Manual IP + “AVR IP Zone 1” trick doesn’t seem to work any longer (this is a really newly-released receiver, so I don’t know if they’ve changed something).
When I pick the auto-detected AVR-X2400H and try the Test Remote, I get a message saying that “Authentication hasn’t been set up”. Unfortunately, I have no idea what authentication is desired.
You can add the Denon as a receiver and as a media player (I did add them both). When you add it as a mediaplayer, you have to enter your HEOS account as username and password.