1 - Are your Linux server and your PCs on the same network ? > If not, open ports and create redirects
2 - In your PCs, is the WOL + S5 standby via network activated options in the bios and in the network card settings under Windows (for eg. WOL enabled) ?
3 - Check the presence of the "AllowWakeFromS5" key (DWORD - 32 bits) with the value 1 in HKEY_LOCAL_MACHINE\SYSTEM\CurrentcontrolSet\Services\NDIS\Parameters\
4 - Are your PCs completely powered off (not in standby or hibernation mode) ?
Note : The WOL must already work with a third-party tool on the same network as your PCs... If not, check the settings.