Figured out how to make the ASUS router Zen Wifi AX work with Roon Arc:
Go to settings in the ASUS Router app.
1.Enable UPNP and port forwarding under Air Protection.
2. Go to WAN settings and go to Port Forwarding. Search for your Roon core from the list of connected devices. Then add a new port and fill out the empty fields for port range with the port number you see in Roon under ARC.
That worked for me. I initially activated port forwarding and UPNP only under Air protection and that alone didn’t work. Step 2 is critical.