Is AT&T Uverse your ISP by any chance? If not, then contact support as mentioned above , or refer to this thread, where this was solved for AT&T Uverse users,
Recently my Internet connectivity has been intermittently going down for a few seconds and then it restarts. (I’m working with ATT on fixing that) But every time it goes down my hub goes offline and stays offline even after the Internet connectivity is restored, usually in a few seconds. I’ve waited many hours for it to come back on its own but it doesn’t. The only way to get it back online is to power it down and up. It seems that this is a new issue with the hub and that in the past it has recovered on its own. Is there anything I can do to fix this? Very often it goes down while I’m away and I can’t reboot it for hours.