@Alexandre.
Thanks. I have not found any place to change any of the settings. When I go to http://192.168.68.1 (-> "Advanced") I can see that the guest network is not enabled, but I have no way of changing that. Also, if I was able to enable it, I would like to change its name.
Also, my modem also supports Wifi, but is there any way to configure Deco to extend the network of the modem (same SSID), or will the WLAN of Deco always be a separate network (different SSID, different subnet)?
EDIT: got it. I had not finished the installation in the mobile app, after I did that I was able to change authentication to none and also change the SSID. No need for guest network. I didn't find a way to extend the existing WLAN, but that is not a big concern.