Well if your behind a router, really you need to get an IP tunneler (not sure if thats actually the right term for it, but thats what I call them) like Hamachi, so you have a seperate IP address, that people will be able to connect to you with.
It's just not easy. You need the other person to have the car that you want to drive so they see it. People are in different time zones. Some can't host. Some Racer versions aren't compatible with other Racer versions.. etc.