If you find that you are unable to connect to a corporate VPN, this could be caused by VPN Passthrough and NAT blocking the connection. You will need to five into your router settings and disable VPN Passthrough.

As an example, we will use a D-Link TalkBox 2800 router to demonstrate the settings. If we review pages 54 and 60 of the manual below we can see that enabling IPSec and L2TP Passthrough and disabling the IPSec configuration in the Application Level Gateway (ALG) should allow VPN connections to pass through the router: -