Look at my picture from here: http://community.nethserver.org/uploads/db8506/original/1X/3757dc0597ed5a467ee84c18421ef91a91d8b9d1.png
(Sharing good Nethserver installation combinations SME with explainations - #5 by fasttech)
It is what do you need.
As @robb said:
You will connect NS RED NIC to your modem (PPPoE in my picture).