Just got 6 Google Chromecast Audio units and set them up to use with Spotify. My config is RB3011 with 2 hAP ACs configured as APs. WiFi connects well, with a stable connection. However, the Chromecasts don't all show up in the Google home app. My 2 APs are configured using CapsMan. UPnP is enabled on the RB3011, and set the bridge as an internal interface. I turned off CapsMan and went back to my old manual configuration, and all was well - all Chromecasts are visible in the Google Home app. Spotify connections are solid. Turned CapsMan back on and the problem returned. I searched the forums and found that I need to turn on Client to Client forwarding in the DataPath tab of the CapsMAN configuration for then network I'm using. Then all was well.
1) Turn on UPnP at the router, with the main bridge set as an internal interface
2) Turn on client to client forwarding in CapsMAN
Hope this helps!