NAD-3305 Error Message Kept Showing Up

My family and I switched to att&t as our internet provider a few weeks ago, and tonight while I was using my laptop the internet suddenly stopped working on all my devices. No problem I thought, I'll just reset the modem and see what the problem is. I probably reset the modem about 8 times until it finally started working again, 3 hours later. I kept having the nad-3305 error message appear, but I checked my broadband wire and it seems fine so I'm not sure what the problem is. Especially since one minute it's working fine and the next thing I know my wi-fi keeps telling me internet is unavailable. The worst part is there's no service out so I can't call customer support, and I can't look up anything of course so I had to go into town to use my school's wi-fi. If anyone can shed some light and what might have caused it and how I can fix the issue next time it happens, I would greatly appreciate it.