5G Authentication error occurred.

Hi. I just purchased Nighthawk AC1900 WiFi Cable Modem Router model C7000v2. It was working fine until 3 days ago. 5G network stop connecting. I got 2 msgs. 1st "Failed to obtain IP address" & 2nd "Authentication error occurred." - l'm stucked on that 2nd one. 2.4 works just fine. Anyway, called Netgear was on the phone for 5 hrs and was told to buy new one. Called Comcast was told the same. For some reason l don't believe l have to buy a new one or exchange it. When l was renting from Comcast (router/modem) l would get the same msgs at least once/ twice a month. They would reset wifi signal and l was good to go until next time. This Netgear Nighthawk model C7000v2 is brand new. I just think they do not know how to fix it. Any ideas???