mDNS in WAN/LAN configuration

I have OpenWRT chained to a TP-Link router using the default firmware. What I would like to do is place IOT devices on the OpenWRT network, and have them be available to the TP-Link router network.

I have already installed Avahi, configured it, and setup the appropriate firewall rules though I'm not able to ping devices from either side.

Does anybody have any idea as to how I would go about this?