[MovedGoalPosts]

Routers do always have some impact in the speed of a connection. If nothing else they add an extra "hop" into the path the internet packets need to take.

I wonder if your router is on the same ethernet speed port setting as the modem, in otherwords it is not sensing 10meg when it should be sensing 100meg? Is there a more up to date firmware, or third party firmware available for the router?

As this is now clearly a router issue, thread moved to networking.