Port mirroring does not work

I want to perform port mirroring with command:

iptables -t mangle -A PREROUTING -j TEE --gateway x.x.x.x

Note that x.x.x.x is ip that I want traffic to mirror to that device.

It works for a while but sometimes it looks like port mirroring stopped.

What should I do to make it work all the time after booting openwrt router?
Thank you