You have to ensure that the version of that router is 100% supported first, and I think it's in the list of devices that are currently having problems flashing DD-WRT....
But where did you find these info " I think it's in the list of devices that are currently having problems flashing DD-WRT...." please???!!
There is no serial port on the router
"Serial Port
In the pictures above there are schematics for soldering an Serial port. The serial runs at 115200 8N1.
To get into the U-Boot prompt, you have to type >tpl< while booting."