To connect at 300Mbps your wireless client must support 802.11n speeds up to 300Mbps as well. For 802.11n performance you will also need to use WPA2 AES encryption. Using a legacy form of encryption will cause it to fall back to legacy 802.11 modes.
After you have logged into the Router click on Configuration > BasicSetting > Wireless. For 802.11n performance select WPA2-PSK under Authentication. For encryption select AES. Set the Pre-Shared Key. The Pre-Shared Key must be at least 8 characters long. Please remember or write this down. Any wireless device connecting to the Router will be prompted for this key.