Situation

Having multiple WAN interfaces and only one can be used for DNS query.
** Can be used for fortiguard too. **

Solution

config system fortiguard
    set interface-select-method specify
    set interface "wan1"
end

config system dns
    set interface-select-method specify
    set interface "wan1"
end

Check

[1]
# FortiGuard連線測試結果
diagnose test update info

[2]
Enable System > Feature Visibility > Local Out Routing
FortiGate WebUI > Network > Local Out Routing