Use Mac connected to cable router (no WAN) as the Gateway of the LAN
Internet Sharing

Open System Preferences, under the Sharing pane, you can configure Internet Sharing:




Share your connection from: select your 3G USB modem
To computers using: Select the interface that your router is attached to (I assume ethernet0)


With that set, you will need to configure your router to use the iMac as the gateway (by plugging it into the WAN port or somehow configuring that in the router's UI).