There are cases when Administrators need Management Interface access over WAN especially while performing Remote Administration. At the same time, you should consider limiting the access only to specific Public IP addresses, change default https port and do not under estimate security threats like brute force attack, password guessing attacks..etc.
Follow these steps to configure Fortinet Firewall and secure its access over WAN
Step 1: Allow HTTPS on Management Interface
On GUI, Network > Interfaces, on Administrative Access section, allow HTTPS
Step 2: Permit Public IP Addresses
On GUI, System > Administrators, enable Restrict login to trusted hosts and specify your Public IP addresses from where you will access. Do not forget to add your Internal hosts, otherwise you lose connectivity to firewall from Internal hosts.
Step 3: Change default https port to 444
On GUI, System > Settings > FortiCloud and change the default https port from 443 to 444.
You should now be able to access your Fortinet Firewall https://x.x.x.x:444 over WAN.