Hi all!
I had fun trying to figure out why IPv6 wasn't working, and didn't have much luck finding a solution in one place, so I thought of sharing my findings.
So the network setup is [ISP Router]---DHCP---[Openwrt BPI-R4]
For reference the ISP Router is the Salt Fibre Box X6 (Swiss Provider) but it should work for all similar XPON routers with CGNAT providers (which makes IPv6 a necessity).
So here we go:
Interfaces > lan > DHCP Server > IPv6 Settings
Interfaces > wan6 > DHCP Server > General Setup
Enable DHCP server but click on ignore this interface - we only need the IPv6 settings.
Interfaces > wan6 > DHCP Server > IPv6 Settings
The picture should look the same as with the lan, BUT also tick the "Designated Master" at the very top. (I will add a screenshot once I'm not a new member and are allowed too 
)
After you apply all changes and refresh your DHCP clients you should get IPv6 prefixes directly from the ISP Router. In my case SLAAC is working only.
I hope that helps someone in the same situation!