By default, the router is configured for auto channel selection, 802.11b/g/n, and auto 20MHz/40MHz. Unlike the early days of Atheros "Super G," there isn't a way to force the router to use 40MHz channels. Atheros' Clear Channel Assessment ensures that the 40MHz channel is used only when it won't cause interference.
You can't achieve 300Mbps phy rate without the Client and AP supporting 40Mhz connection.