2. Anything happen during the time it worked and stopped? Power outages, storms, etc?
3. What have you tried to fix it?
4. Does the ethernet work and just not the wifi?
5. Can you log into the gui via the browser to see if the radios have been disabled?
6. Did the wifi on/off button get pushed? You can push and hold it for 2 seconds to turn on/off the wifi.
Here's your support page with manuals, firmare, and installation instructions. Keep in mind, there are multple versions of the R6400 so select your correct version from the drop down.
