Ipv6 global address always lost after reboot

My ott device is X96 MAX+, the gigabyte network works fine on my device. I enable ipv6 on my device. I create a file " /etc/sysctl.d/ipv6.conf ", add the belowing content "
net.ipv6.conf.all.disable_ipv6 = 0
net.ipv6.conf.default.disable_ipv6 = 0
net.ipv6.conf.eth0.disable_ipv6 = 0 ", after execute “systemctl restart systemd-sysctl” as root. the device will get a global ipv6 address immediately. But after a while. the device will lose ipv6 global address. Is there anyone know how to keep the ipv6 address