Modem problem

The manual at https://secure.xfinity.com/anon.comcastonline2/support/help/faqs/wireless_gateway/HOW5220_Wireless_Gateway_3_UserGuide_06_19_15.pdf says:

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Wiring Problems
If the Telephony Gateway begins flashing all its lights for more than 10 seconds, this indicates a problem with the telephone wiring — the red and green wires may be shorted (touching), or there may be undesired voltage on the lines.

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@Julian000001 wrote: But I don’t have telephone line or use it.

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Make sure nothing is plugged in to the Tel1 or Tel2 jacks.

If the device has a backup battery, unplug the power cord and remove the battery for a minute or so.

If you can't get it working, take it and its power cord to a Comcast Service Center or Xfinity Store and swap it out. Check locations and hours at https://www.xfinity.com/support/service-center-locations.

If power-cycling it doesn't work, you can try hard resetting the device to factory defaults by pressing and holding in the recessed reset button on the rear for 30 seconds. This can sometimes clear up some weird behavior issues. You will lose any customized settings, and they will need to be re-configured from scratch if you do so. Good luck !