Serial Recovery
From DD-WRT Wiki
Routers that have a serial port can often be recovered by using a serial adapter. This is an alternative to Jtag.
Serial ports are normally a four or five pins on the router motherboard. Usually, you have to solder to pads or remove solder from the holes and install a four or five pin header.
You connect to the router with the programs hyperterminal or putty.
Power cycle the router and while it is booting hit control C quickly. You should get a CFE>
The most common command you will use is the "nvram erase" command.
To flash the firmware
stop the boot with ctrl-c Enter this command "flash -ctheader : flash1.trx