Answer

XXXXXXXXXX,

Just some clarification first...
The cable modem you have has the ability to go at 100mbs, your router has the ability to go at 1000mbs. When you have the router plugged into the cable modem using a network cable, it will be connected at the lowest speed of the two... so, it will use the 100mbs.

So, I'm guessing that where you see the 54mbs is on your wireless device (maybe a laptop)? This will be determined by the network card that you have installed in the laptop. If the network card has the ability to go at the speed of Wireless N (which is 270mbs max), then it will try to connect at that speed. However, reality kicks in, and the speed will be reduced based on wireless signal quality, distance from router, etc. So, the laptop network card will scale down the speed until it gives you the most reliable connection.

I hope this clarifies how this stuff works...

Please let me know if you have any questions.