Sharing a drive on different Wi-Fi Networks

That would be an issue with your wifi router, not your computer. That said, the point of a guest network is normally to provide guests wifi without letting them have access to the devices on your main wifi network.

Sounds like your previous setup was actually wrong, to be honest. Maybe there was a firmware issue with access leaking across wifi networks for which your router manufacturer pushed a fix.

As correctly stated in the reply above, this is not a problem with the macOS and iOS configuration.

It’s entirely the Wi-Fi router.

Your Wi-Fi router controls what connections are permissible (can be routed) between a guest network or guest VLAN and the main network or main VLAN.

IP routing is centrally involved here, and client-to-client connectivity (sometimes called client isolation) can also be involved where that feature is separately controllable.

I am here assuming the rationale for the guest Wi-Fi is for a more easily changed password, and for a network “off” switch, as the guest network is not otherwise providing much in the way of security. This I would typically not expose my main network to nor expose a core Mac to a guest network.

More help with this? Check with the folks that know the details of your particular Wi-Fi router best.