If you put it in N-only mode I think that you loose support for G mode.
you need to put it in mixed mode so that it can use N and G,
the wireless router that I have at the moment supports G and B so I have it in mixed mode so that an older wireless device I have can still get on the network.
enabling two modes lets the fast mode enable and it's top speed and the slower mode work at it's top speed, neither of them interfere with each other.