openwrt documentation on IPv6 is either very brief or not noob friendly. For anyone trying to configure ipv6, please go through the below links (ordered) first.
https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol
https://en.wikipedia.org/wiki/Radvd
https://en.wikipedia.org/wiki/DHCPv6
https://openwrt.org/docs/techref/odhcpd
https://mkaczanowski.com/ndppd-ipv6-ndp-proxy/
https://forum.openwrt.org/t/clarifying-ipv6-luci-terminology-ndp-proxy-ula-prefix-server-mode-relay-mode-hybrid-mode/18743/16
when ipv6 connectivity starts working and if you are wondering why the hosts are not getting GUA updated when wan6 interface gets a new prefix from ISP, refer the below link.
https://www.6connect.com/blog/is-your-isp-constantly-changing-the-delegated-ipv6-prefix-on-your-cpe-router/
ps - updated google cache link for above since original link isnt working.