You must open incoming port(s) to access FortiGate over the Internet.
HTTPS is the first port that is needed. Other ports are optional depending on what features are enabled. See FortiGate open ports in FortiOS Ports and Protocols.
- Go to the VPC where the public-facing subnet belongs for the FortiGate.
- Select Firewall rule, then Add firewall rule if the required port is not open.