The FortiGate default LAN subnet conflicts with the Starlink router LAN subnet, the FortiGate WAN interface cannot obtain the allocated IP from the Starlink router.
The Starlink router does not provide any option to change its own DHCP Server IP range, ‘192.168. 1.0/24’ is hardcoded.
If the FortiGate LAN subnet cannot be changed, the VDOM can be used to separate the WAN and LAN networks as a workaround.

The config example is shown below:

• Enable multi-VDOM mode:

config system global
    set vdom-mode multi-vdom
end

• Create the ‘Internal’ VDOMs:

config vdom
    edit Internal
next
end

• Assign interfaces to the VDOMs:

config system interface
    edit “wan1”
        set vdom “root”
    next
        edit “port1”
            set vdom “Internal”
        next
    end

• Configure the VDOM link:

config system interface
    edit “vlink10”
        set vdom “root”
        set ip 10.0.0.1 255.255.255.252
        set allowaccess ping
        set type vdom-link
    next
        edit “vlink11”
            set vdom “Internal”
            set ip 10.0.0.2 255.255.255.252
            set allowaccess ping
            set type vdom-link
        next
    end

• Configure the default static route to the Internet in ‘Internal’ VDOM:

config vdom
    edit Internal
        config router static
            edit 1
                set gateway 10.0.0.1
                set device “vlink11”
            next
        end

 • Configure the firewall policies from ‘Internal’ LAN to the Internet:

config vdom
    edit Internal
        config firewall policy
            edit 1
                set name “Internet”
                set srcintf “port1”
                set dstintf “vlink11”
                set action accept
                set srcaddr “all”
                set dstaddr “all”
                set schedule “always”
                set service “ALL”
                set nat enable
            next
        end

config vdom
    edit root
        config firewall policy
            edit 3
                set name “InternaltoInternet”
                set srcintf “vlink10”
                set dstintf “wan1”
                set action accept
                set srcaddr “all”
                set dstaddr “all”
                set schedule “always”
                set service “ALL”
                set logtraffic all
                set nat enable
            next
        end

Test PC (192.168.1.100) can access to internet in the FortiGate LAN subnet via Starlink router WAN:

Tracing route to dns.google [8.8.8.8]
over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms 192.168.1.99
2 1 ms <1 ms <1 ms 10.0.0.1
3 1 ms <1 ms <1 ms 192.168.1.1
4 7 ms 6 ms 6 ms static-209-87-245-1.storm.ca [209.87.245.1]
5 7 ms 6 ms 6 ms google.ip4.torontointernetxchange.net [206.108.34.6]
6 7 ms 7 ms 7 ms 108.170.250.241
7 7 ms 7 ms 7 ms 216.239.35.233
8 7 ms 6 ms 6 ms dns.google [8.8.8.8]