As others have said Marantz are only Roon Tested this means they only connect via Airplay over the network, this is wired or by wireless. Roon tested is not the same as being Roon Ready. It just means it’s been tested and works using a different connection method than Roons default streaming… If you don’t see it as an airplay endpoint in Roon then something in your network is blocking discovery. Here is where mine shows
However there is a Roon community extension that I use with my Marantz 40n that allows Roon to connect to it using a a UPNP bridge that uses the inbuilt Heos streaming allowing much better quality connection up to 192/24 and is how I use my Marantz with Roon every day. You do need to run the extension on another computer or nas though. It’s also 3rd party so not supported by Roon. It’s how many users get to use Roon on non Roon Ready network streaming devices. I am very happy with it myself and in no rush to get a separate streamer as it works and sounds great.
Failing that it’s adding a Roon Ready streamer, you can get the Wiim pro for £150 and connect it to the coax input on the Amp.