NB5540 - VPN Passthrough Problems Guide

Problem/Symptom: 

In some cases, you may be having trouble with your VPN pass-through connection.

Resolution:

The size of the MTU has to be increased to 1492.

Go to the Admin page and specify the MTU to be 1492 (default is often 1400) also ensure IPsec and PPTP pass-through are enabled.
Click Apply. This should resolve the problem.