I think you should post your whole configuration following this:
http://forum.mikrotik.com/t/forum-rules/173010/1
Besides other things, it is important to know which device is it and which OS version it is running, the Iliad (pseudo) guide should be about a RB4011 with - possibly - an older version of Ros, a full export will have this info at the beginning.
Are you sure you started “clean” of default configuration?
There could be settings in a default configuration that could be at least part of the issue.
Not an expert at all in IPv6, but shouldn’t the “main” route:
/ipv6 route
add disabled=no dst-address=IP6_NETWORK gateway=WAN routing-table=main
be pointing to the whole internet?, like:
/ipv6 route
add disabled=no dst-address=::/0 gateway=WAN routing-table=main
And I am not at all convinced that gateway=WAN is correct, compare with the example here, where the actual IP address of IP6_TUNNEL_GW is used:
https://forum.fibra.click/d/45711-configurazione-iliad-ftth-net-neutrality-su-mikrotik-rb4011?page=11