Chromecast problem after adding 2nd AP AC Mini to network

I’m having my problems getting my Chromecast to work after adding a 2nd AP AC Mini to my network. Previously, it worked fine when there was just one AP AC Mini in the network.

The setup: multi-story home, a Balance One Core managing AP AC Mini’s, each LAN port on the Balance One Core is connected to a different wall ethernet port in the home. Previously had just one AP AC Mini connected to one of these wall ports and supplying Wi-fi for the entire home. The Chromecast worked fine then. Recently added a 2nd AP AC Mini to another one of the wall ethernet ports. Both AP AC Minis use the same settings profile in the Balance One Core AP Controller. Have 3 SSIDs, each with its own VLAN, but I had the Chromecast connect to the same SSID before and after I added the 2nd AC Mini. That SSID has the Layer 2 isolation box unchecked in the settings.

When I go set up for the Chromecast, I get an error page saying that the Chromecast is connected to the network but there’s issues with communication between the smartphone and the Chromecast. The help page the error page directs to talks about turning off AP isolation and/or turning on UPNP. The Chromecast worked fine when I had just one AC Mini in the network and UPNP was turned off then as it is now.

Any help with this issue is appreciated. Thank you.

Edit: to add, the smartphone that I’m using to run the Chromecast setup is connected to the same SSID as the Chromecast.