Upload Firmware via Recovery Utility
Set your NIC card to static IP 192.168.1.12, Subnet 255.255.255.0, Default gateway192.168.1.1 (Static IP guide)
Put router into recovery mode by holding down the reset button while powering up the router, wait until the power led is blinking slowly, then release the reset button.
In the Asus Recover utility Access the router’s built in recovery mode by issuing 192.168.1.1 in browser field.
Flash the initial DD-WRT build (e.g.) Asus-RT-N66U_18946_mini.trx build and upload.
When completed wait until the router re-boots on its own. This takes at least 10 minutes.
Put router into recovery mode by holding down the reset button while powering up the router, wait until the power led is blinking slowly, then release the reset button.
Flash the final DD-WRT build, could be any of:
18946 Mega
18946 Big
19342 Mega
19342 Big
When completed wait until the router re-boots on its own. This takes at least 10 minutes.
Put router into recovery mode by holding down the reset button while powering up the router, wait until the power led is blinking slowly, then release the reset button.
Select Reset NVRAM defaults when it completes (should be quick), in the browser field, type in http://192.168.1.1/do.htm?cmd=nvram+commit. When done press enter. When complete select reboot and wait 5-10 minutes for the router to reboot and start building its layers.
Now you should be able to access via web gui 192.168.1.1 and it should be DD-WRT.
Also check your CFE version.
If cable is not working, try wifi.