How to disable Fast Startup using a Group Policy?
It looks like there is no Administrative Template for managing this setting; as documented, Computer Configuration\Policies\Administrative Templates\System\Shutdown\Require use of fast startup
can only be used to enforce it, not to disable it (WTF?!? It's already enabled by default... they could at least have gone a bit further and turn this setting into a true on/off switch!).
The only available way to disable Fast Startup (outside of using the GUI) is by setting the following Registry key to 0:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Power\HiberbootEnabled
This can be done using Group Policy Preferences and it effectively disables Fast Startup; of course, setting it to 1 would instead enable it.
And yes, disabling Fast Startup fixes the problem of Wake-On-Lan not working.