A mac computer (OSX Mavericks) is connected to a TP-Link router via WiFi, as well as a number of other devices. The TP-Link cable router (no ADSL modem), is not connected to a WAN. The mac computer has access to the internet via USB 3G modem. I would like, if possible, to offer my mac's internet connection to the LAN and have the mac act as the gateway of the LAN.
In short, replace Internet access of the router through WAN with mac's access to internet through USB 3G modem.
USB3G -- Mac -- Router -- Device1, Device2, etc
i.e. give internet access to Device1, through Mac's internet access of USB3G. It has to pass through the router, therefore using OSX's internet sharing is not an option.