Three common ways to deal with dual stack networks on openwrt, including ebtables (layer2 passthrough), odhcpd and nat6 (nat66, similar to classic ipv4 nat)
Copy them to anywhere you want on the router. Run one of the .sh file after booting, or add it to rc.local.
Each .sh file among the three will disable the other two automatically when called with ---.sh enable.
Check the code for more details.
Bridge the WAN and LAN port, and uses ebtables to block any non-ipv6 packets between them.
Suggested by OpenWRT. May not work if your ISP's device doesn't follows the IPv6 standards well.
Anti the concept of IPv6, but really stable. Last method.