Problem with D6400

[LS11]

I activated the auto sleep function of wireless router in the evening; when the device is reactivated, in the morning, the 2GHz band does not work, forcing me to reboot the device from the web page of the router (if you turn off and turn on the device still does not work).
The 5GHz band is operating normally. Since the seller has already replaced me the device with a second (which had the same defect) I don't know what I have to do.
Any suggestions? 

[M-B]

Check you have the latest firmware installed (1.0.0.52).

If this is OK then consider performing a factory reset and set it up again.