If you’re relying on automatic configuration for port forwarding in Roon Settings → ARC, then please post a screenshot of the diagnostic snippet and failure message that you see there. It should indicate “Not Ready” if there’s an issue.
Please also share the name of your internet service provider, since many ISP implementations outside of Roon’s control will require additional steps for ARC to function via cellular data.
You can also try out Tailscale, a third-party app you can install on your RoonServer machine and phone to allow for easy ARC access.