Hi !
Not that i'm a specialist, but i would suggest that u put a NIC in your server and use the USB cable line as the second interface.
So that your LAN is connected to the NIC and the WAN connection goes trough the USB line.
Setting up your server as "server & gateway".
Be sure that u have the proper drivers for both the USB interface connected, as the NIC that you intend to install.
Rgs tmn