A you might want to post in the correct forum. You posted in the app section of the forums and not the cable modem routers section as thats what the D6400 is. You'll get more responses.
https://community.netgear.com/t5/Cable-Modems-Routers/bd-p/home-cable-modems-routers
When you post in the correct forum, make sure you include more info. Here's whats recommended.
Firmware on your device:
More details on the description of the problem. You stated ethernet ports not working. What is working? Can you connect to it? What are the led's on the panel doing (colors, blinking, etc).
What have you tried to do to correct the problem. (such as reboot, hard reset the device).
here's the link for the support page for your device. There is a manual there that you can look through for troubleshooting or how to reset your device.
https://www.netgear.com/support/product/D6400.aspx
