kevinds advice is the right way to go. If your devices work on the 5GHz band, it should work much better than the 2.4GHz band. If you're still using the 2.4GHz band, the range will be brutal because of the interference.
The RT-N56U supports having separate ssids for the 5GHz and 2.4Ghz networks. If you setup a router ssid for 2.4GHz and router-5g ssid for the 5GHz network, you can check whether you can see the 5GHz network on your devices.
I ended up buying one of these on eBay for bluetooth and 5GHz support on the laptop, which required flashing a modified BIOS to work (to get around the whitelist):
»ark.intel.com/products/5 ··· ual-Band
In our condo, I was able to get ~90mbps over the 5GHz network, with the router at one end of the condo and the laptop at the other.