How do I schedule a reboot in FortiGate firewall at a certain time from remote ?

For monthly reboot, only the trigger will differ. E.g. trigger monthly on 1st of each month at the 22:10:

config system automation-trigger
    edit "ScheduledReboot_trigger"
        set trigger-type scheduled
        set trigger-frequency monthly
        set trigger-day 1
        set trigger-hour 22
        set trigger-minute 10
    next
end

The rest is the same as in any kind of reboot.

Some more example stitches: https://github.com/yuriskinfo/Fortinet-tools/tree/main/Fortigate-automation-stitches

BTW: For the future, it is better to create a new post, as this one has been already marked as “Solved” many will not look at it.

Regards