Fortigate port-forwarding using dynamic IP (such as PPPoE and L2TP dialers).
Go to Policy & Objects -> Objects -> Virtual IPs and click Create New
Enter the VIP name
Choose the internal interface
Leave External IP Address/Range with all 0.0.0.0
Enter in Mapped IP Address/Range the internal IP address of the server
If you want all ports to forward to this address click OK else click Port Forwarding checkbox and enter the external and internal protocol and ports.
Here in my example its 192.168.10.150 (server IP) and I’ve mapped TCP port 80 to 33000.
Go to Policy & Objects -> Policy -> IPv4 and click Create New
Choose WAN interface as incoming interface,
Choose LAN as outgoing interface,
Choose server VIP we have just created as Destination Address.
Select NAT on and select all other required parameters (AV, IPS, Web Filter etc.)