I'm playing a random map with many islands. My largest island with about 10 cities has no iron. I have cities on other islands with iron, and rush built a harbor so that my main island could use it. Even though my main island has several harbors, and the the city with the iron has a harbor, the main island does not have access to the much needed iron. My trade advisor says that the city is unconnected and needs a sea route. What else do I need to to, to establish this sea route?