Fortigate 40F behind Draytek 3912 performance issues

Hi everyone,

We’re running into consistent performance and internet connectivity issues when using a Fortigate 40F firewall behind a Draytek Vigor 3912 router.

The WAN connection (with a dedicated public IP address) is managed and delivered by a third-party IT provider. So far, they’ve only been able to make the setup functional by using a double NAT configuration.

Unfortunately, we don’t have administrative access to their VPN router, meaning all network and routing configurations can only be modified on our Fortigate side. This limits our ability to optimize or troubleshoot the setup beyond our local network.

Has anyone experienced similar performance degradation or connectivity problems in a comparable setup (Fortigate behind Draytek / double NAT scenario)?
Any advice on best practices or alternative configurations that could help improve throughput, latency, or overall stability would be greatly appreciated.

Setup overview:

• ISP / third-party provides WAN and dedicated IP

• Draytek Vigor 3912 (managed by third party)

• Fortigate 40F (internal firewall / our control)

• Double NAT currently in use

Thanks in advance for any insights or troubleshooting tips.