Some ISP only offer you a /64 prefix. If you want to add an OpenWrt router between the ISP router and your LAN while granting IPv6 access to LAN devices, you need relay. OpenWrt's built-in odhcpd(odhcp-ipv6only) can handle DHCPv6 relay. To enable this feature, you need to enable relay mode for ra and ndp for the wan6 (The upstream-facing IPv6 interface) and lan:
Reboot your router and your LAN devices should get IPv6 addresses.