SRC_IP >> port2 (FG) port1 >> DST_IP
You should test telnet from the host to the destination, not from FortiGate CLI.
In the session list you will see if the NAT takes place correctly for the source/destination IP.
If no session in the list with the correct filters - check the debug flow - it will tell you why.
- Toss a 'Like' to your fixxer, oh Valley of Plenty! and chose the solution, too00oo -