When you configure DDNS (Dynamic DNS) with ADSL connection in FortiGate you may receive the below error message if the selected interface had private IP address.
Warning: This interface has a private IP address (<IP Address>) which may not be publicly accessible.
To resolve this you have to change the WAN service type on your ADSL router and change the addressing mode on FortiGate interface. First log in to your ADSL router note down the User Name and Password and delete the existing settings and add a new WAN service.
Select Bridging as WAN Service Configuration.
Save the configuration.
Log in to your FortiGate unit and choose addressing mode as PPPoE and enter Username and Password.
Now you will see the detected public IP address in the interface.
Now it is able to configure DDNS settings.