Monoprice PID 8743 Wireless N ADSL2 Modem and Router 802 11N WPS Function 2T2R 300Mbps - Manual (Page 70)

ADSL2/2+ Router
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To enter virtual server interface, click “NAT”-> “Virtual Server”. And
then click the “Add” button to add rules for accessing the server.

Use Interface: It indicates which WAN connection the configured
rules are to be applied to. When there is only one configured
WAN connection available, system will select it automatically.