I configured the guest network exactly as explained in https://openwrt.org/docs/guide-user/network/wifi/guestwifi/guest-wlan-webinterface and it works fine.
However, I found that guests may need to connect to my TV, which is on the local LAN with static IP address, not the guest LAN, in addition to going online.
Is it possible to allow the guest LAN to access additionally a single device of the main LAN? how to do it?
My workaround now is to create another wifi belonging to the local LAN and to activate this wifi only upon need: this avoids having to disclose my main WiFi password and I still get to limit the access.
Modifying this second guest wifi as requested above would be preferable but I have no idea how.