In addition to having the modem with either a battery or plugged in to a UPS, you also need to have the signal booster plugged in to a UPS. The signal booster should look like a transformer with a coax cable plugged in to it and might be located near your TV cable box. Without this your cable modem will continue to work, but it won't get a strong enough signal to keep your service up and running. In my house I have two UPSs, one that the cable modem is plugged into and another that the DVR and signal booster are plugged into so that it can continue to record shows if there's a brief power outage.
I am having the same issue in Savannah, Ga. Comcast has Power Supplies/UPS back-ups mounted to Utility Poles, most of the close to the area Distribution Amp. Thr batteries in the Power Supply/UPS maybe bad or other issues with their equipment. The problem is how to report the issue to Comcast.. They believe the equipment will send s low battery singal if the batteries are bad and that signal will Not be sent if the equipment (Power Supply/UPS) goes dead when AC power is lost. You may need to go the the main Comcast office in your area to try to report the issue, the call center is useless about this issue. Good Luck