Thank you for reaching out. The error you’ve posted, in the context of your mesh network setup, can usually be resolved by taking one of the following steps.
Enable UPnP across all your daisy-chained modems/routers (The Dasan terminal, the Asus router, and the Asus node). If UPnP is not available in all of these routers, or you still receive an error, try step two below instead.
In your router and modem web administration pages, create identical TCP port forwarding rules. These rules should point to the static IP address and listening port that are listed in Roon → Settings → ARC.
If you continue to experience difficulty, please post screenshots of the web administration page of your routers. Be sure to obscure your public IP address for security.
Additionally, who is your internet service provider? We’ll want to verify if they have implemented CG-NAT on your account.
I was running the core on QNAP. QNAP has implemented a firewall and had to allow ports to be opened for ARC. I don’t own a Synology, but if you have a firewall try turning it off. Second, if you can connect to ARC from outside your home LAN, your firewall settings should solve the problem.
You can install Tailscale on your Synology and on your mobile phone. This is a VPN solution based upon WireGuard – no need to open ports of fiddle around with firewall settings. ARC on your mobile will see the Roon Core as it it were in the same local network, even when your are on the road.