That is kinda a strange requirement when you pay a lot extra to get a internal modem.
If the router has the feature it would allow you to change one of the lan ports into a wan port. I know this has become common on DSL modem/routers. You would have to dig through the manual but all the DSL devices have one of the lan ports marked wan/lan. This device does not but that does not mean a lot you would have to check the manual.
Although they could provide the feature I doubt there are many people that spend the extra money and then only use the router part.