With an MC7304 successfully installed and working on Windows 8.1, Skylight can see it but there are no virtual COM Ports, e.g. the AT COM port. What is the process for enabling the COM port?
Problem is that by plugging it into Windows 8.1 I think it has put the unit into MBIM mode which does not have any serial ports, which then means you cannot get it out of this. You need to contact your commercial channel to get some support as I think there is a tool to get it ut of this mode but is not generically available.
Have you plugged the unit into a Linux or Win 7 system to see if you can see any com ports? If not then the above will be your issue.
You can fix it using the available Windows tools too. KiernanV tested and described the necessary steps here: