If I have understood your problem correctly, you are trying to reach public IPs from the Intranet through the Fortigate firewall. The firewall is in transparent mode. So all traffic that is allowed will depend upon the policies you have put in the firewall. In transparent mode the firewall is invisible to the network as all interfaces are in the same VLAN and no traffic can pass unless you apply policies. So please check this out. You can try using Port Address Translation (PAT) instead of NAT, or maybe switch to IPv6 if your applications and other OS support IPv6.
Hope this helps.