Client computer has an IP address of 1.1.1.33 on the subnet 1.1.1.0/24

Router “A” sits between the client computer and the FortiGate (in Transparent mode) with the IP address of 1.1.1.1 on the client’s side of the router and the IP address of 192.168.1.211 on the FortiGate’s side of the router.

Use NAT to assign addresses from an address pool of 9.9.9.1 to 9.9.9.99 to traffic coming from gateway of 192.168.1.211.

To enable the return traffic to get to the original computer, set up a static route than assigns any traffic with a destination of 9.9.9.0/24 to go through the 192.168.1.211 gateway. As long as the session for the outgoing traffic has been maintained, communication between the client computer and the external system on the other side of the FortiGate will work.