I have had this same issue with other equipment and Comcast as the ISP.
It required multiple frustrating phone calls to technical support and
persistently stating that I wanted to speak to a highest level technical
support team in their area. Finally...
It may becoming an issue due to cascading routers or something similar.
Placing your modem / gateway into bridge mode may also resolve the
issue. Lastly, I would check the firmware versions on each device.
