This article shows you how to set the IP address of a Fortinet Fortigate appliance via the command line and set it to transparent mode.
To connect tot he Fortigate’s command line, open the web interface and scroll to the bottom of the Dashboard (there’s a command line widget) or connect via the console.
In the command line run this command:
config system settings
set opmode transparent
set manageip 192.168.1.1/255.255.255.0
set gateway 192.168.1.254
You will then have to reconnect to the Fortigate web interface with the new IP address before you can make any other changes.