The speed of a wireless connection depends on numerous factors. The 300 Mbps is just the theoretical maximum, and in fact you'll never get that speed, even under ideal conditions. Some of the factors that affect the speed are:
- the mode which is set in the router wireless configuration
- the capability and configuration of the wireless interface in the equipment at the other end of the link
- the distance between the router and the receiving equipment, and what obstructions there are, such as walls
It may be possible to improve the link speed, but I'm afraid I don't have the knowledge to advise. Someone else may be able to help, but in the meantime it would help to know what wireless device is in the receiving equipment (laptop or whatever).