I have checked the configuration again and tested further.

I think, that the router recognizes all SSIDs as the same Wifi, therefore i can only enable 1 SSID, as the first SSID overrides the second and third (Guest overrides IOT and Music SSID, IOT overrides IOT), so that only the first SSID is shown.

The Guest and Music SSIDs work on their own. With the SIOT Network there seems to be a configuration issue as a connection is not possible.