Your D-Link DIR-645 CAN get 300 Mbps but it sound like the link speed it configured at 20 Mhz. You need to put Auto or 40 MHz only, you are able to get 300 Mbps max.The wireless configuration are in setting, use manual config then you will see the 20 MHz. Change it to Auto 20/40 MHz. Hope it help.
reply to piggygirl I would also set for Mixed G and N or N only if you can. Check what your devices support.