Problem: You are using a 802.11n compatible router like the FritzBox 7270 and a WLAN Card like the Intel® Centrino® Advanced-N 6200. However the router and the Windows OS shows only 54 Mbit/s instead of the expected 300 Mbit/s.
Solution: To use 802.11n with 300 Mbit/s WPA2 is required for that. To fix the issue you need to reconfigure the Windows OS configuration and configure WPA2 with AES e.g. instead WPA with TKIP. Once the connection is established you should be able to see 300 Mbit/s in the router overview and in the Windows OS.