Portforwarding between gates

[lowlander]

Hi,

We have two fortigates deployed.

WAN <-> Gate 1 <-> L3 switch <-> Gate 2 <-> VLAN B, VLAN C

------------------------------------<-> VLAN A

Gate 1 routes all traffic to VLAN A, by adding a route to Gate 1 that routes al traffic to the IP address of L3 Switch;

The layer 3 Switch routes all traffic from VLAN A back to the Fortigate;

We want to isolate VLAN C adding a gate in front of it, and having control with Gate 2 for this VLAN. One interface for Gate 2 is connected to VLAN B and another to VLAN C;

On Gate 2 we have created a VPN portal. The management of the gate is redirected to another http and https port. The VPN portal listens on the interface IP of VLAN B;

We want to publish the portal to internet.

We have created a port forwarder based on a VIP with corresponding policy in Gate 1, were we define the external IP address. This forwards traffic to the interface address of VLAN B on Gate 2;

When we are on VLAN A connected to Gate 2, we are able to access the external IP address and are forwarded to the VPN web access portal.

When we try to connect via a different ISP, the forwarding seems to fail;

What is the best way to troubleshoot this issue ?