Mqtt config with .things file not working

Add-ons Bindings
,
5

Here, the channel belongs to to the Thing - because it/they are enclosed in curly braces { }
It is effectively
Thing blah { channel, channel }

But
The Thing does not belong to the broker Bridge. No curly braces.

You can either use curly braces to nest Things in Bridge

Bridge mqtt:broker:mymosquitto "mqtt Broker" [host="192.168.100.11", secure=false, clientID="openHAB", port=1883 ]
{
    Thing topic zuhause "mqtt Zuhause"
    {
           Channels:

or you can tell each Thing which broker it belongs to

Thing mqtt:topic:mymosquitto:zuhause "mqtt Zuhause" ...