Hi @Deon_Tanzer,
Thank you for your post.
The MultipleNatFound snippet above indicates that there is a second, redundant network address translation layer (NAT layer) between RoonServer and the internet.
This can either take the form of a second active router or your service provider’s own Carrier-Grade NAT (CG-NAT).
You mentioned you have two routers active - in that case, port forwarding can’t automatically configure via UPnP/NAT-PMP.
You’ll instead need to access the port forwarding section of each Deco router separately and create a TCP-protocol rule pointing to the IP address and port number listed in Roon Settings → ARC. Make sure this rule is identical in both router administration settings pages.
Alternatively, you can use the built-in Tailscale network interface in your Nucleus One. This will bypass port forwarding entirely, so you wouldn’t need to configure anything in your routers.
Please see here for steps specific to Tailscale: