I'm not quite clear on which device has the WAN connection, and which device you want to initialise the connection.
Anyway if you look at my old thread here:
https://archlinuxarm.org/forum/viewtopic.php?f=48&t=12623It gives one way of doing it. Basic idea is you'll need one side or other to bring up the usb gadget support. I used libcomposite - as its the newer system. Using gether is another option, and you'll need to google that.
Then you need to decide what kind of usb ethernet you are bringing up, I did CDC - mainly becuase i'm a linux guy and had no wish to support windows machines.
Once you have the usb ethernet set up - you should be able to do $this->bbcode_second_pass_code('', 'ip a') on both machines, and they should show a $this->bbcode_second_pass_code('', 'usb') device.
Then you'll need to bring the ethernet up. Suggest you have one side as the master. On that you'll need to configure the interface to come up with an IP number, and probably want to start a dhcp server on that machine, and handing out the IP address you want the far end to use.
At that stage, with any luck it will bring up ethernet on both sides.
Then on the side with the WAN you'll have to worry about routing, you can do forwarding, or NAT; depends on how widely you want to distribute the knowledge of the Ip sub net you have set up between the devices. Me, I propagate that subnet across my whole LAN, which means I can log into my machine from anywhere on the LAN. However for WAN I go out via NAT.
Sorry if this sounds complex, it is because it is. What must be, can not be avoided. Here trying to set up a totally new network connection via usb ethernet. That network needs its own existence, and that what needs doing.
Its actually a path worth following, because when you get it to work, you'll understand networking far better.