[edited] Router advertisements were not enabled, DHCPv6 cannot work like that.
Only question left: where can I see the delegated IPv6 prefix?
This output from "netstat -rn6" is not entirely clear to me: why is it Static (S), what is it blackholed (B)?
2a02:1234:1234:8400::/56 link#5 USB lo0
This is what worked for me:
ifctl -6pi pppoe0