Okay, that’s good progress then.
So as long as you have a String Item in openHAB that changes when it receives an update from your MQTT broker, then the topics I linked to should get you much closer.
Unless you are trying to filter parts of that message and do different things.
In which case this topic might help you?
I’m really a Velbus guy, so anything else is pure guesswork and a great deal of trial and error.
(Until some kind soul shines a light on a solution)