If you have a business grade router like a Ubiquiti EdgeLite then you can configure this. If this is a generic off the shelf consumer router then no you cant make a Lan port into a Wan port.
Now what is happening to make you think your Wan port is dead. Did it work just fine and then stop working, or did it never work. I ask becasue I wonder if it is not just a configuration issue.
What ISP do you have? No ISP uses ethernet as the actual data medium from their hub to your house, you must be using cable, phone, or fiber which needs a media converter (which is falsely called a modem) to convert it into RJ45/ethernet.
Also if you have AT&T uverse or other fiber company that lan cable might not be carrying ethernet protocal data and...