The first diagnostic message indicates that your router is functioning with both IPv4 and IPv6 address formats. Most commonly, your ISP will have that dual capability in place with additional implementation that might interfere with ARC, like carrier-grade address translation (IPv4) or some sort of unique prefix handling (IPv6).
What I recommend is disabling UPnP in your router’s web administration page and creating a manual port forwarding rule in the router’s port forwarding or security settings. This rule should be TCP-format and point to the IP address and port assignment listed in Roon → Settings → ARC.
Additionally, how many routers do you have in your setup? If you have a second router downstream from the ISP modem, you’ll need to make sure any router capabilities in the modem are turned off (Bridge Mode).