Same here. Our 2.4 band went down January 22 (two days ago). As a result, we had to move the WiFi for every wi-fi dependent device and computer to 5 band. In the meantime, our video doorbell no longer works (SimpliSafe requires the 2.4 bandwidth—no exceptions). So, until the mystery is solved we have no way to see who's at the door.
Our modem model is the Technicolor XB3—dual band WiFi 802.11 ac
What can we do to remedy this?
Have you first tried power-cycling / re-starting the gateway ? No joy ? 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 the latter.