When using PPPoE, it works just fine if you have settings as follows:
- pppoe-client property add-default-route=yes
- ipv6 settings property accept-router-advertisements=yes-if-forwarding-disabled
- ipv6 dhcp-client property add-default-route=no (this is default setting)
Things get tricky when WAN interface is “plain” ethernet with DHCPv6 client running … one can try with add-default-route=yes which may or may not work as desired. But in this case, proper solution is to set ipv6 settings accept-router-advertisements=yes … but you’re saying that his doesn’t work correctly.
But in any case it’s very tricky if one has multiple WAN Interfaces. Or if one has LAN Interfaces with clients sending out RAs (which properly working clients should not).
You can try to block routing advertisements in firewall raw, check this post for slightly more info about that.