there are several reasons why you get a speed of up to 54mbps only.
1.since the adapter has 3 antenna ports, you will need 3 antennas to be able to fully utilize the card's capability. connecting to 2 antennas is fine but the card won't be able perform its fullest capacity.
2. wireless authentication - for testing purposes try no authentication, see if the speed exceeds 54mbps or not.
Configure the Wi-Fi client device's profile to use Wi-Fi Protected Access* (WPA2-AES or WPA2-TKIP). More info here.
Hope this is helpful.