The only way to expose this to say the web (that's a WAN) is to put this server into the DMZ. Did you try the DMZ?
PS. There's another issue with the file sharing system you are trying to use. It's generally accepted it doesn't work across networks like that. This is why folk use FTP and other file services. This is NOT an offer to fix Microsoft's implementation you are trying to use.
I have a raspberry pi 3 with Windows 10 IoT installed as the operating system using Noobs to download the OS files. I have a 2TB external hard drive connected and the device is connected to my wifi router with an Ethernet cable.
I have set the static IP address, set to boot headless, and created two user accounts on the device using Power Shell on my desktop PC running windows 10. I can map a network drive on my local LAN using the local IP address and the drive letter using the following notation "\\xxx.xx.x.x\e$" where e is the drive letter of the USB port where the storage is mounted.
I tried to mount the network drive from another windows 10 PC on another network (not at my house). When I select the check box "log in with another user" and hit finish, I get a login pop up and enter my user name and password that I set up on the pi. It says "attempting to access the folder" or some such message. after a few seconds (10-20) I am presented with the login popup again. It does not say that the password is incorrect or that the location cannot be found or anything.
I have forwarded several ports on my router to allow access but I am unsure if I have forwarded the correct port. I am also unsure if the port is the reason I cannot seem to be able to connect. Please help.

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