Ok...
Very limited searching and reading suggests that the WAN MiniPort is a software construct. Essentially a little helper program that sits between the OS (Windows) and the device drivers and facilitates certain types of communication.
For example, if you were using a VPN (Google: Virtual Private Network) you would (from what I understand) most likely have one of these programs running between Windows and the drivers translating the normal packet information that Windows is sending into the correct format for the established VPN.
All this is based on literally a couple of minutes Googling. The fact that there's not a lot of information about other than MS developer stuff suggests it's not something that normal users would normally get involved with.
Obviously I could have interpreted everything I read completely wrongly, so no promises that I actualy know what I'm on about here