I want to use AirPlay to cast from my Mac Studio in one subnet to my Apple TV in another. What can I do to ensure that the mDNS requests are able to leave their local subnet?
I’ve tried setting up traffic rules using 224.0.0.251 but have not had any luck. It appears that one solution, avahi, is already installed on my router but I don’t know if any additional steps are required. Suggestions?
I would not recommend the changes to get it working, but here is a solution that might help:
https://community.ui.com/questions/Airplay-across-VLAN-How-to-do/0362cb7f-f38c-43ba-b10e-c2e5cc9dbe16#answer/acb73608-5980-4cd0-8dd7-d6125e753dc5
yes you can do this with avahi 
here are some links I advise to check:
but with the current changes in OpenWrt I don’t know if this still work under wireless, I believe you have to check the checkbox multicast to unicast in luci wireless settings.
so far there are ports 5353,8008,8009,5443? what you want to do is allow such ports from zone to zone, from what I understand is that clients talk to 224.0.0.251 which then talk to 0.0.0.0 (the router), and from there it talks back, but you still have to allow zone forwarding for discovery.
my guess is to just try it with the examples in the url above for now and if it works slowly change/expand them for your needs 
in the past it worked for me but now I’m not sure if it still works.