I am getting extremely frustrated. In December, I made the switch from Century Link to Comcast because Century Link kept increasing their rates. I was with them for many, many years and NEVER had I experienced so many connection problems! Almost daily I see this massage, "Authentication error occurred" when I try to connect my phone to my WiFi. I never had to consider increasing my data plan through my phone carrier until now - proof that my WiFi keeps disconnecting. I've emailed about this (with no response) earlier this month and now, again. Honestly, I'm about to switch back to my old provider if this isn't resolved. Higher prices, but assured connection.
Try a factory reset. Stick a paperclip into the reset hole in the back of the modem for at least 30 seconds. Then when it finishes rebooting...everything will go back to the default network name and password as provided on the labels(side and bottom). Assuming its a Comcast gateway, you'll either see XFSETUP-×××× or, a HOME-xxxx wireless network name broadcasting among other wifi names. Then try connecting to them.