The Arris NVG510 is paid for.

If their customer buys their own modem instead of leasing it from the
ISP then the ISP won't support that modem. Well, they might but they
will likely charge for fixing something that isn't their property. The
customer should start hunting around for a new modem after checking with
their ISP which are their recommended brands and models.

I just got back from Christmas dinner at the folks I was helping. User
said he fiddled with the router and PC until somehow the PC started
receiving Internet. Unfortunately he isn't computer savvy and has no
idea exactly what he did to fix the issue. The broadband light is a
solid green now. I checked his Android and it was still receiving
WiFi, and his cellular data was enabled. Thanks to all who replied,
sorry for the anti-climatic closure.