Note that the “in between” router needs to serve up DNS – that’s the whole point of doing it. My Linksys/DD-WRT router (which is what I used) does not do that by default – it passes DNS requests up to the main router.
I had some truly bizarre stuff happening on Saturday when I added a new laptop to my network. The “in between” router was set up as 192.168.1.x, and the Arris is set up for 10.0.0.x. Somehow the laptop grabbed 192.168.1.1 as the DNS server, rather than the Arris. The result was that the laptop could resolve IPV6 addresses, but not IPV4. That took a while to figure out… I mention this just in case anyone else has weird network issues when they have an “in between” router on their LAN.
BTW I cannot figure out a way to disable IPV6 in the Xfinity/Arris modem…