If you enable Use default gateway on remote network on your Synology Router, all its network traffic is routed through the remote VPN server. That means your DNS hostname will point to the IP address of the VPN server you are using instead of the address of your Synology Router. You therefore cannot access your Synology Router via DDNS.

Within a local network

If you are on the same LAN as your Synology Router:

• Connect directly to your Synology Router using its original LAN IP address.

From an external network

If you are connecting through the internet:

• Use Synology QuickConnect (refer to this article for detailed instructions) to reach your Synology Router.¹
• Inquire whether a port on the gateway of the remote VPN server’s network can be forwarded to your Synology Router.²