Hey, Sadly iOS does not support TAP mode, and will not work with Roon Core over VPN.
From openvpn:
Why doesn’t the app support tap-style tunnels?
A: The iOS VPN API supports only tun-style tunnels at the moment. This is a limitation of the iOS platform. If you try to connect a profile that uses a tap-based tunnel, you will get an error that only layer 3 tunnels are currently supported.