My Deco is set up as an access point (via ethernet) to my Comcast Technicolor XB8 Gateway. But the Gateway assigns what looks like ipv6 DNS addresses, and the Decos say there is no internet access due to private DNS addresses.

2001:558:feed::2

2001:588:feed::1

The XB8 says the DNS mode is DHCPv4 with the usual Comcast DNS servers: 75.75.75.75 and 75.75.76.76.