In case someone else arrives here. It turns out, from Swisscom technical support, that the Guest WiFi network simply does not work on the WLAN box. WLAN box is only 5Gz and Guest Network is only 2.4Gz !!!
I have enabled the Guest Network feature on my Internet-2 Box. I also have a WLAN Box (extender). The main WLAN network performs correctly from (in range of) either box. But the Guest network only works via the Internet-2 box. When only in range of the extension box it says it has a self assigned IP address and no Internet access. Would be nice if the Guest network worked everywhere the main network worked. Is this a feature, a bug or a config error.