Hi,
It seems that you have not enabled DHCPv6 on your router.
Please, navigate to Network -> Interfaces -> Edit LAN interface -> DHCPv6 server -> IPv6 settings and enable DHCPv6. You can try to set everything to hybrid, DHCPv6 mode to Stateless + stateful, and enable 'always announce default router'.
Now, your router should provide your local devices with IPv6 addresses that are needed for communication.
Kind Regards,
Andzej