IPv6 DHCPv6 Relay help

[FrAllard]

Hi,

I have a friend that need to install OPNsense behind his ISP device. The device need to be kept for phone and TV purpose. However he would like to use OPNsense for his own network. His provider is giving him GCNAT IPv4 + Native IPv6 to the ISP box. Once we connect OPNsense on a lan port of the ISP box the WAN receive a DHCPv6 address in the same /64 as the isp box. In fact the WAN address is being given a /128 I don't know why. The ISP box status seems to say that it has received a /56. I can confirm that trafic for every /64 under the /56 is being routed to the isp box.

Enough of the backstory! Here is the problem. We tried WAN DHCPv6 and ask for a /64 delegation and LAN Track WAN Interface. It's a no go, LAN doesn't get an IPv6.

We tried to enable DHCPv6 Relay on the LAN by giving one of the /64 address from the /56. I packet capture the WAN while forcing a DHCPv6 client on the LAN to request an address and the relay packets are being sent to the box, but no answer...

Which IP addresses should we setup for the WAN and the LAN and eventially the LAB and enable DHCPv6 Relay and have it work?

I'm also wondering if we'll even be able to have a separate LAN and LAB because I'm wondering how are we gonna tel the isp box that it need to route the trafic of that /64 to the WAN on OPNsense?

Here are an example of what the IPv6 addresses look like at the moment.
ISP Box status tell us we have a delegation looking like that : 2001:1234:5678:99::/56
ISP Box LAN address: 2001:1234:5678:9901::1/64
ISP Box deliver DHCPv6 in that range.

We tried giving the OPNsense LAN IPv6 : 2001:1234:5678:99bb::1/64
Then enable DHCPv6 Relay on the LAN interface. The capture on the WAN was showing all the right address and was showing relaying to 2001:1234:5678:9901::1 as instructed.

We looked everywhere at example on how to successfully setup DHCPv6 Relay on LAN.

Can anyone please help?