Status code “504” together with “Did not find UPnP service with WANIPConnection on network” suggest that UPnP might not be working
Try to disable and then reenable UPnP in the web administration interface for the router directly upstream from your Core. Turning it off and on again might help
If not, disable UPnP and try to manually open the port in your router’s port forwarding configuration. Make sure the IP/Port matches the Port listed in Roon → Settings → ARC
Check for any VPNs or Firewalls that might be interfering
If you have a separate modem: Check if your modem is in Bridge mode
If you continue to experience difficulties, please provide the following information:
What is the make and model of your modem and/or router?
Do you have any additional network hardware, like additional routers or managed switches?
Who is your internet service provider and what is your geographic region?
If you have a separate modem in front of your router, is it configured in Bridge Mode so that it operates only as a modem or, alternatively, do you have the ports forwarded on both?