unable to connect vpn from forticlient

Hello  I have created the IPSec Vpn on the Fortigate D300 but when i try to connect the from the forticilent from the out off the network it's showing me "unable to reach tunnel gateway/Server Policy" The firewall is configured behind the Cisco router so no public IP is available on the firwall And i'm using the NAT on router to map the Port number "8900" Please also suggest to me which port number shall i use on the csico router for the IPSec VPN  Please help