ok. I think I know what you are trying to achieve. But I am unsure if you are completely aware of the airplay concept.
Let’s just start from the beginning.
All your devices (Marantz, RoonCore and iPad) are in the same network (even though the iPad uses a wireless connection). This is a requirement.
Let’s leave Roon out for a moment: Are you able to use Airplay to stream music from your iPad to the marantz (for example if you watch a youtube video or play something in iTunes)?
If that works, check if the Marantz is set-up as an Airplay output in the Roon audio settings:
In my settings you see the AppleTV as an Airplay device. You should see there (hopefully) the Marantz. If there is an “enable” button next to it, the Marantz was not enabled yet for RoonServer. Please do so.
When using your iPad, it should connect (wirelessly) to the Roon Server (or core). As the Marantz is registered as an output you should be able to select it in the Roon Remote as an output. You can set the output in the bottom right corner next to the volume icon. Do not enable Airplay on your iPad over control center: The iPad is not streaming the music! In case of Roon, it is just a remote and will tell Roon Server to stream to your Marantz.
Airplay has nothing to do with wireless or wired. It is a protocol that can be used (wired or wireless) to stream audio in your local network.