Hi @MaartenMechanic 

Thanks for sharing your story and config note here with us. I have labeled it as KB. I believe this is helpful for others who might run into the same situation.

Appreciate your effort and time for this little article.

Thank you @Clive_A ,

Additionally,

Although Delta/Caiway still states in their documentation they do not support IPV6 yet, they actually do. I've configured it like this: 

IPv6 

Omada Controller -> Site -> Settings -> Wired Networks -> Internet

IPv6: Enable
Connection Type: Dynamic IP (SLAAC/DHCPv6)

Get IPv6 Address: Automatically
Prefix Delegation: Enable
Prefix Delegation Size: (64)

DNS Address: Get from ISP Dynamically

MAC Address: Use Default MAC Address

For LAN DHCP use:

Omada Controller -> Site -> Settings -> Wired Networks -> LAN

Edit your existing DHCP Server, scroll down and expand: 

Configure IPv6
IPv6 Interface Type: SLAAC+Stateless DHCP

Prefix: Get from Prefix delegation

IPv6 Prefix Delegation Interface: WAN

IPv6 Prefix ID: 0

Address Prefix: (This will populate automatically)

DNS Server: (I have entered them manually, you can find them in from the Router/details tab, simply from your devices list) 

Now my DHCP client devices in my network obtain an IPV6 address automatically and immediately. 
 

Check if it works for instance with: https://test-ipv6.com/