Most likely your ISP is natting your address, and you will not receive a public IP.
So in that case you need to have a dialup connection.
FortiGate 200A/B, 224B, 110C, 100A/D, 80C/CM/Voice, 60B/C/CX/D, 50B, 40C, 30B
FortiAnalyzer 100B, 100C
FortiAP 220B/221B, 11C