I've been trying to setup my Google Home Mini but when I do it either says it can't communicate with my phone halfway during setup or gives me an error talking about AP/Client Isolation. I can see that the Google Home connects to my network and is given an IP from the DHCP server (and I can ping it). The only thing I see that would come close to AP/Client Isolation is Client To Client Forwarding found in datapaths in capsman. I also have UPnP enabled as that is a requirement for the Google Home. Google documentation can be found here and here.

AP isolation is Default forward disabled in wlan1 or client to client forwarding in Capsman. If you're using local forwarding maybe you should enable them both, otherwise without local forwarding enable only client to client forwarding.

I have the capAC, I know forwarding for devices is the default, in other words, new clients are not blocked as the firewall list is not checked on the AP.
Not sure what you mean by AP isolation?
Is there additional functionality and if so do you mean, client to client isolation within a WLAN??

AP isolation: Wi-Fi clients can't talk to each other without passing through the router, it's the function that blocks casting.
In MK isn't called like that, but is default forward.
Default forward:
Flagged: no AP Isolation
Unflagged: AP Isolation, you can't use Chromecast

In CAPsMAN it's called "client to client forwarding" and flag works in the same manner.

I did get it working, not entirely sure what caused it to start working but enabling both Local Forwarding and Client To Client Forwarding seems to work.

Local forwarding makes wlans to forward traffic like you're not using CAPsMAN, you can see that because you need to bridge wlans when in local forwarding.
Maybe it was horizon that was blocking Google Home and cast, I don't use it and neither local forwarding and I can cast.