I had been able to find my Cast devices while on different subnets but it stopped working recently… not sure why… in search of a solution I found a specific answer for those using pfSense.
This worked for me with Google Cast but may work with other discovery components that typically don’t work on different subnets.
You can install the ‘avahi’ package on pfSense and once enabled on the subnets you want to cast between, HA should pickup and find all devices. Not sure if this has been mentioned before but thought others may find it useful.
Reference here: https://www.reddit.com/r/Chromecast/comments/454fsi/chromecast_across_subnetsvlans_pfsense/?st=iurq9hzy&sh=f168b206