To install DD-WRT from ASUS firmware (using a pin or appropriate device) hold reset button in while power on router.
Wait for slow power light to flash. TFTP the 'dd-wrt.v24-25974_NEWD-2_K2.6_mini.bin' to 192.168.1.1
Wait several minutes then power off router. Turn power back on while holding the red WPS button (approx 20 seconds). 
This will reset its nvram. After fully power up you should be able to access router webif at 192.168.1.1
You should then install the 'dd-wrt.v24-25974_NEWD-2_K2.6_mega.bin'
Best I remember there were some issuses with port 4 not working until you done the 2nd DD-WRT install using the webif/GUI ...
... so best to be plugged into one of the two middle ports (2 or 3) to do this initial install.

on DD-WRT this router will show as a RT-N12B