Traffic to Guest Network not possible

I like to use the guest network for all equipment that may hurt my privacy. That can be guests but also equipment produced anywhere in the world that I may not trust and requires to use my wifi credentials. This equipment is ip mains-switches for lamps in my house, measuring apparatus, what ever.

As this potentially dangerous equipment cannot reach my private network I stil want to control this equipment by reaching it from my private wifi AND wired network.

There is the option "Allow Guests to see each other and give access to my local network". If I switch this option ON I can reach the equipment on a fixed IP on the Guest Network. If I switch the option off I cannot reach the equipment on the Guest Network anymore.

Anybody any idea how to reach equipment on the Guest Network without allowing equipment on the Guest Network to reach my private network? Many thanks for your support.