@Zeppi 

Sure this is possible - just make sure your main WiFi and the Guest WiFi have different names and all devices are connected to the proper one.

  @Hoamboy

Thank you - i will try again, but what you mean with "... all devices are connected to the proper one." 

  @Zeppi 

The question is what do YOU mean with "but in the 2.4 network many Smarthome devices go offline."
As long as you setup everything correctly and connect every smart device to the proper SSID ("WiFi") then everything should be fine.

Make sure every device get its IP address and don't mess around with VLAN settings or WPA (security) settings.

In most cases it's way easier to setup a new WiFi (SSID) from scratch than spending time to find the mistake in previous setups.

Saying this because setting up a WiFi in Omada is really simple and straight forward.

  @Hoamboy 

Ok - as Fan from FCB we can write german
Ich hab jetzt das zweite WLAN eingerichtet - nicht als Gast-WLAN.
es scheint zu laufen. Beim ersten Mal habe ich als Gast-WLAN eingerichtet und die Shelly-Smarthome Teile verloren die Verbindung zum ersten WLAN.
Ich beobachte es weiter.