This article describes that it is not possible to set the negotiation option for the WAN interface to manual. The option only shows ‘Auto’
Scope
FortiGate v7.2.7 and later, v7.4.x, FortiGate-10xF.
Solution
By default, the WAN interface speed option is set to Auto. But speed negotiation options should also be available.
In addition, for some Internet Service Providers that require manual setting and auto-negotiation to be turned off, this might be an issue for the mentioned hardware and firmware.
Below is a sample of the interface with no other speed option for WAN:
Resolution: Upgrade to v7.2.9, v7.4.5.