It certainly should work, either on wired Ethernet or WiFi. It could be a network discovery issue. On rare occasions one of my Roon Endpoints won’t appear in the Roon Core settings. Restarting Roon usually sorts it immediately, if not try restarting your router.
Ok, new problem. Now I cannot seem to regulate the volume on the network player. I cannot do that from Roon, only from my amplifier to which the network player is connected.
An amp with analogue volume control is unlikely to allow Roon to control it. I’m not too familiar with your Denon, but I believe it’s similar to the Naim gear I use in that you can link the streamer to the amp with an additional cable, and use the streamer remote or control app to adjust the volume.
I don’t know if your streamer has, in addition, a digital volume control that can be used within AirPlay?
If the amp has some network control then one user has written an extension that can control certain models of Marantz and Denon. You could check it out in the tinkering section. Roon cannot control you amp on its own it has no connection to it. It can only control connected Roon Ready devices such as all in one systems, Devialet, Sonia, NAD, Niams uniit range as they all have amps in built. It can also control preamps in DACs if they expose the control to Roon and it’s been tested but this is a small percentage. Other options are to add a harmony hub system which can use ir and again their is a user extension that opens this up to mix into Roon. I use this all the time in my 2nd system and its works brilliantly allowing Roon to control the volume of my amp. Airplay sometime has access to a digital volume control in the device it’s on to get certification as its part of Apples spec but being digital volume it can effect SQ. Naim have it on their streamers but don’t recommend to use it but not heard this being on an amp before.