Hello @AHE 

Thanks for asking in the community.

When the Archer C80 is working in AP mode, the guest network and the main belong to the same network, and you cannot disable the guests to access the local network.

The main reason to have the main network and the guest network when it works in AP mode is to distinguish the two, as some customers prefer not to share wireless credentials with guests.

Imagine when the router is working in AP mode, and it is allowed to configure isolation between the main and the guest, the devices connected to the guest will not be able to access the internet, as they cannot communicate with the gateway in this configuration.