Hi, I like to explain it step by step

1. Usually this C7 router is used as a router. So we set up guest network to allow guest for Internet access. But prevent the guests from accessing other LAN hosts.

2. If the option "Allow Guest to Access My Local Network” is selected, it means we allow the guests to access other hosts in LAN. That is what you don't want.

3. Now, you are using this C7 as a pure access point, which is another case. An access point is like a wireless switch, it is totally transparency for all LAN hosts and just passively forwarding the frames between the LAN hosts. If the guest network clients want to communicate with your main router (not C7), you must enable "Allow Guest to Access My Local Network”, otherwise the guests are forbidden to talk with your main router.

In short, if the guest want internet access, they need to talk with your main router, if they need to talk with your main router, they need to access the local network.

To block guests to access local network is not the only purpose of guest network. You can configure Wi-Fi name and password and set a schedule for the guest network.
Anyway, if you use it as an access point, you have to allow guest to access local network, which means that guests can see your music and pictures in the same network.
But you can use it as a wireless router to aviod the contradiction.

Thanks for the replys. In my scenario I will not be able to use the C7 as a wireless router, only as access point. I am going to deactivate the guest wifi anyway if it does not make a difference to the normal wifi. Less to configurate...