I understand you’ve endured an extreme wait time for a response to your inquiry, so on behalf of the team, please accept my apologies and my thanks for your patience.
The 502 status code indicated in the diagnostics you’ve shared suggests that either a second layer of network address translation or a network security setting is interfering with your Core’s ability to reach ARC via your two routers.
Try to enable UPnP/NATPMP in the web administration interface for the router directly upstream from your Core (this will be in the web administration page of the Tenda unit).
Try to manually open the port in your router’s port forwarding configuration.
Make sure that the port listed in the Roon ARC tab in “Settings” (usually this is port 55000 or similar) matches the port assigned for the port forwarding configuration in the Tenda and the Vodafone routers.
Double check for any network security that might be interfering, or any firewalls on your Core.
Check if your Vodafone modem is in Bridge mode or equivalent.
If the above steps aren’t helpful, then please let us know. You may need to confirm with Vodafone if they have implemented carrier-grade NAT in your region, and we’ll provide some questions you can pass along that will help clarify the situation.
I’ll be watching for your response. Thanks again @jon_fernandez.