Follow the guidance posted by @Suedkiez (link below), and use the following settings:
- Mode: single
- Service name: RoonARC
- Protocol: TCP
- WAN interface: ALL
- Public port: what you see in Roon > Settings > Roon ARC, e.g., 55000
- Source IP: any
- Private IP: the IP of your Roon core
- Private port: same as publiuc port