You can connect to both 2,4 GHz and 5GHz your WiFi card supports both, the question is which standard and frequency does you router/wirelles access point work.
You are probably on 802.11g standard which has the speed up to 54 mbps but will in most cases work with less speed (depending on the distance from AP and number of devices on it)
You can look here to see more about differences between standards
If you router is set to 802.11n, some claim that 802.11n works slower in mixed mode (see the thread here). The claims are following
- Just putting the AP to mixed mode slows the speed
- The speed gets slow down only if b clients are connected
- The speed slows down if b or g clients are connected
- The speed slows down not based on mixed protocol but encryption (if you use WEP,WPA or mixed WPA/WPA2 encryption), actually this isn't just a claim Intel states that data rate will not exceed 54 Mbps when WEP or TKIP encryption is configured, it's 802.11n cards will fall down to 802.11g standard, because 802.11n prohibits using wep or tkip on networks, so if both your client and AP are N check if it's configured to use WPA2