Why does my guest router lose IPv6 if I reboot the first router?

I have two OpenWRT routers running OpenWRT. If I reboot the first router my second router IPv6 stops working, all the IPv6 details are still there in the second router, nothing changed, just all the devices stops working with IPv6, I can even see the devices with IPv6 addresses. There should be a renew option if the IPv6 is not working.

Renew where?

The devices behind router 2 don't know router 1 rebooted, even more so, if you're double NATing.

Renew on the second router.

Bring wifi down and up again, on router 2, to force a renewal on all wireless clients.

Just tried to restart wifi and it did not work.

Update: Restarted WAN6 interface and IPv6 came back, I dont know why this is happens. If I reboot the main router the second guest router should see the main router is disconnected and should renew WAN6.