I have set up Guest Network WiFi and have disabled both options "Allow guests to see each other", and "Allow guest to access your local network". Even so, with my mobile phone (Android using the app Fing) connected to the guest network I can see the PC connected via LAN cable and various mobile devices connected via WLAN to the non-guest network, which surely rather defeats the object of a guest network.

I am using my Archer C80 router in Router Mode, not Access Point Mode.

Although I could always see the LAN-connected PC the mobile devices were sometimes not visible.  For test purposes I have been shifting the mobile devices around guest network and non-guest network, so I am wondering if that is part of the problem and there is perhaps some cache or list which has to be updated, and if so, how to force the update. I have tried completely disabling the guest network and reenabling it, but that has not helped.

Has anyone else had this problem, or know a solution? 

Edit: rebooting the rooter did not change anything.