Accept_ra_rt_info_max_plen not found

I've created my own firmware for openthread and openwrt. I have to set accept_ra_rt_info_max_plen at /proc

echo "64" > /proc/sys/net/ipv6/conf/all/accept_ra_rt_info_max_plen

But "accept_ra_rt_info_max_plen" does not exist. I don't found which Kernel Module is missing. Has someone any tipps for me?

Greetings, Tom