VPN Fortinet and EdgeRouter Lite v1.9.0

Hello team,

I'm new with Ubiquity device, i try to find some documentations or tutorial on internet but i didn't find the solution yet.

I have a problem to configure an IPSEC VPN between a EdgeRouter Lite and a Fortigate 110C.

I attach the topology to the case.

To sum up :

the Edge router is installed behind another firewall. This firewall has a nat rules that nat the public IP address to the Edge router behind. We have a local subnet between the firewall (192.168.28.0/24) and the edge router.

I would like to configure a VPN between the edge router and a the fortigate installed in main office.

The edge router is installed on the office, and all the computers can acces internet normaly. Only the VPN part doesn't work.

Here is the current VPN configuration on the Edge routeur :

vpn {
    ipsec {
        auto-firewall-nat-exclude enable
        esp-group NANCY {
            compression disable
            lifetime 86400
            mode tunnel
            pfs enable
            proposal 1 {
                encryption aes128
                hash sha1
            }
        }
        ike-group NANCY {
            dead-peer-detection {
                action restart
                interval 30
                timeout 60
            }
            key-exchange ikev1
            lifetime 86400
            proposal 1 {
                dh-group 2
                encryption aes128
                hash sha1
            }
        }
        ipsec-interfaces {
            interface eth0
        }
        nat-networks {
            allowed-network 0.0.0.0/0 {
            }
        }
        nat-traversal enable
        site-to-site {
            peer IP_WAN2 {
                authentication {
                    mode pre-shared-secret
                    pre-shared-secret ****************
                }
                connection-type initiate
                ike-group NANCY
                local-address IP_WAN1
                tunnel 1 {
                    allow-nat-networks disable
                    allow-public-networks disable
                    esp-group NANCY
                    local {
                        prefix 192.168.9.0/24
                    }
                    remote {
                        prefix 192.168.1.0/24
                    }
                }
            }
        }
    }
}

Hope someone can help me.

Thanks for your help.

Regards,

Davvv