IGMP proxy is only needed for IPTV, a specific protocol for delivering audio and video content. This requires a package in your internet subscription. It is not needed for home media servers or live TV services delivered through apps or web apps.
At the heart of IPTV there is the multicast optimization technique. Multicast is well-suited for local networks (meaning local PROVIDER’S network) and specific use-cases, like IPTV, where the same data is sent to multiple recipients from within the provider network.
This is very different from streaming platforms like Netflix or Amazon generally do not use multicast due to the unicast nature of the internet and the complexities involved in implementing multicast at a global scale. The same is true for a home multimedia server like Plex, multicast optimization is generally not necessary for a small number of delivery points like a household or a small community.
If you are trying to do casting then that won’t natively work across subnets because it uses a broadcast approach and broadcasts can not be routed from one subnet (green) to another (blue) or vice versa.
The mDNS addon allows that broadcast limitation to be overcome.