Network isolation in the AX73 does not work in Access Point mode.
Since I have another router, I use this model only for Wifi, thus setting into AP mode.
The Guest Network menu advertises "Create wireless networks for your guests so that you don't need to disclose your main network." but its not true.
On the Guest Network I can still access all my network devices (my NAS, my Raspberries...etc).
There are no option to turn this off.
In this form, the Guest Network kinda useless, only another SSID for the same network.
If I set the router in Router mode, and use the "manual AP" mode, I can isolate the Guest Network, but that way I even cut it of from DHCP, making it imposible to connect to it.