So I had to change routers once in a while and of course updated OpenWRT to the latest release compatible.
As I needed to purge the config I had to reconfigure my routers naturally.
Problem: IPv6 GUAs were not assigned to clients in the home network
I followed the IPv6 troubleshooting to the letter, but it still did not work.
Solution:
Through searching the forums of OpenWRT I finally found a thread where it was mentioned that the wan6 interface should have "DHCP Server"->"IPv6 Settings"->"Designated master" enabled, and exactly that did the trick.