@craigmod I discovered that if I set my router to "passthrough" mode for IPv6, after a minute or so, IPv6 started working properly from my MBP.

My guess is that you want to use the ONU's or ISPs DHCPv6. If the router does it, then it messes up the routing. Just a guess.

Seems like you can do that on a Deco setting IPv6 mode to Bridge?