Can't get traffic mirroring to work

I'm getting started with openwrt. I'd like to setup traffic mirroring for a particular IP on my network to send mirrored traffic to my PC.

I entered these commands:

iptables -t mangle -A PREROUTING -s 192.168.1.<x> -j TEE --gateway 192.168.1.<y>
iptables -t mangle -A POSTROUTING -d 192.168.1.<x> -j TEE --gateway 192.168.1.<y>

where <x> and <y> are the local addresses of the device I want to mirror and my PC, respectively.

I then turned on wireshark on my PC, generated traffic on device <x>, and expected to see packets between 192.168.1.<x> and various internet hosts. But all I see is local traffic, mostly ARP and ICMP and stuff like that. I also see lots of traffic that is generated by my local PC <y>, but I"m not interested in any of that.

the rule appears to be triggering, because I can see the packet/byte counters increasing:

root@DLink2640:~# iptables -t mangle -L -v | grep PRE -A 3
Chain PREROUTING (policy ACCEPT 803 packets, 133K bytes)
 pkts bytes target     prot opt in     out     source               destination         
   56  9343 TEE        all  --  any    any     192.168.1.<x>        anywhere             TEE gw:192.168.1.<y>

All the online advice seems pretty straightforward, so I can't figure out why it's not getting mirrored. What am I doing wrong?

Is there a way to TEE to a local file on the router so I can at least inspect what, precisely, is getting mirrored?

Did you set the monitoring device to see packets not destined for it?

I used wireshark to on the monitoring system (address in my post) to sniff all the packets coming in on the ethernet interface.

I set the filter line on the top in wireshark to ip.addr == 192.168.1.<y>

Is there some other method I should use?