I don’t have that modem/router so i can’t log in and check but you have to change the dns settings. 

This video goes over changing dns on arris routers but not sure if it applies to your device. 

Unfortunately, the YouTube video doesn’t apply to my router. I’m unable to change my DNS settings as described in the video. Any other suggestions?

I don’t have that modem/router so I can’t log into and try it. What actually happens when you try? 

What screens do you see (screensnips help) and/or do you get errors?