Tcpdump on mirror ethernet link

Hi all,

I have OpenWRT installed on a TP-Link C7, and a router from my ISP which support mirror configuration. As I just want to tcpdump the traffic from the mirrored traffic, I thought just to plug it in the OpenWRT and dump the traffic of the interface. However, I cannot seem to get it working even though I have setup an unmanaged interface and set it into promisc mode. What is withholding OpenWRT from showing me the mirrored traffic?

Thanks for your help