Chromecast and plex server with Orbi. Multicast IGMP proxy?

I have 15 Chromecast Audio and 1 Chromecast Ultra. They kept disappearing from the network on my phone also.

I switched off the Wifi, bought an ethernet interface for each of the CCA, and used AV1200 powerline adapters. Now all is well.

Unfortunately, the Chromecasts just don't play well with wifi mesh. I even tried a Velop on sunday - same exact issue. It's much more problematic when you have a bunch of Chromecast audio in a group and suddenly a few disappear. That means you can no longer control the volume or stop casting to those devices (but they keep streaming!).

https://community.netgear.com/t5/Orbi/Chromecast-Audio-and-AP-Isolation-problem/td-p/1389027

https://productforums.google.com/forum/#!topic/chromecast/BlfyzpS3MAE;context-place=topicsearchin/chromecast/category$3Achromecast-audio%7Csort:relevance%7Cspell:false

I'm using Wemo switches to turn the amps on/off for all 15 zones, and having problems with that too - they keep randomly dropping off the Orbi network, just like the Chromecast. They do come back eventually. But it's very annoying when you want to turn things on or off and you can't because your phone can no longer see the "smart" plug. Wish there were smart plugs with ethernet instead, but I could not find any.