That IPv6 forwarding page is strange. IPv6 does not need forwarding.
Anyway, I am trying to find a manual for your router. A Google search shows that it is probably NL localized and probably Asian manufactured for NL ISPs. Are you able to get a manual from your ISP? Their website looks just like one of ours - no help at all. I have also tried searching on the model and not much comes back.
I’m off on holidays for a few days. I’ll be back on dinsdag/tuesday (08/08/2023).