Connect the console cable to the router.
Make sure that your tty device shows up in /dev.
Verify:
ls /dev/ttyUSB0
returns:
/dev/ttyUSB0
The same instructions would apply for /dev/ttyS0 if you used that type of cable instead of a USB console cable.
NOTE: If you are using Windows, your Serial port will simply be called COM1.
Make sure you have putty installed, and start it as root:
sudo putty
NOTE: If you use Windows, launch putty with administrative privileges.
Click open and your session should start.
NOTE: You may need to go into the BIOS of the router to allow it to boot up from the USB.
You also may need to press a key, such as F10 to make sure your USB stick has been detected.
sudo screen /dev/ttyUSB0 115200
NOTE: If the command above fails, try a different device for the serial console.
Other values could be /dev/ttyUSBx or /dev/ttySx (where x is a number, starting from 0).