In order to get ARC working you need either a port forwarding setup or a Tailscale setup.
I’m not familiar with your ISP so there may be another wrinkle. If your ISP uses CG-NAT to supply you with a an Ip address, then Port Forwarding will not work. In this case, you will either have to get your ISP to give you a public ip address that supports port forwarding (and then you can set up port forwarding) or you will have to use Tailscale.
Since you’re using a MacOS RoonServer, a simpler option than port forwarding would be to install Tailscale. This will connect your Mac Mini to your phone automatically without port forwarding.
See here:
MacOS Sequoia has tighter network security than previous MacOS versions. Make sure you’ve gone to System Settings → Privacy & Security → Local Network and toggled on local network access for RoonServer. This will make sure your local network Zones work with Roon.