In DSM's Control Panel > External Access > Router Configuration, the system detects two or more routers in your network.

DSM’s Router Configuration function uses the UPnP protocol to set port-forwarding rules for the router within the same subnet. If you have two routers connected in series, the UPnP message can only reach the child router. This makes the parent router unable to apply the port-forwarding rules.

To verify this issue, you can run the traceroute command to check possible routes in your network.

Run traceroute

On a Windows computer

1. Press Windows key + R and type cmd. Then, click OK.
   
2. Type tracert 8.8.8.8 and press Enter.
   

On a Mac computer

1. Press Command + space.
2. Type Terminal in the search box and press Enter.
3. Type traceroute 8.8.8.8 and press Enter.

On a Synology device

1. Sign in to DSM as root via SSH。
2. Type traceroute 8.8.8.8 and press Enter.
   

If there is more than one hop within the private network IP range, it indicates that your PC or NAS is located behind two or more routers. To allow external access, you will need to configure port forwarding on each of the routers.

Private IP addresses include ranges of:

• 10.0.0.0 - 10.255.255.255
• 172.16.0.0 - 172.31.255.255
• 192.168.0.0 - 192.168.255.255
• 100.64.0.0 - 100.127.255.255

This screenshot shows that both hop 1 and hop 2 are within the private network IP range (10.0.0.0 - 10.255.255.255). This means there are two routers connected in series.

Change one of the routers to bridge mode

If you have two routers in series, for instance, a modem with built-in NAT capabilities connected to your home router, a simple solution is to put the modem into bridge mode by disabling the NAT feature.

Please contact your ISP (Internet Service Provider) for the bridge mode settings.

Configure port-forwarding on both routers independently

If you need the two-router configuration, you will need to manually add port-forwarding rules to each router. For detailed steps, refer to the documentation provided by the router’s manufacturer (e.g., Synology).

Contact your ISP for more information

Different ISPs may offer different levels of service or have different network configurations. Some may not allow port-forwarding because their network is behind a Carrier-grade NAT. Please contact your ISP for more information about external access.

• Synology NAS External Access Quick Start Guide
• I cannot set up router configurations in DSM. What should I do?