John has been upgrading his parents' network with a new router, some smart devices like NEST Cams, etc. He's running into some issues, though. He suspects it's the modem/router combo. Leo says it likely is: you'll need to find the IP address and log into it. Likely Admin/Blank for a user name and password. Then turn off the wifi and DHCP. The DHCP is also doing double NAT, so turn that off. It's called putting the router side into "bridge" mode. Let the modem send the bandwidth to the Google home router and it will handle it all. That should fix it all.
Here's a tech note on it - https://www.pcwrt.com/forums/topic/how-to-disable-wifi-for-centurylink-router/