You’ve shown the configuration for your Broker Thing. Can you show it for your Generic MQTT Thing?
Sounds like it might be worth going over the basics of Thing - Item interaction. There’s now a Getting Started tutorial which might help.
Otherwise, this tutorial goes through the steps of adding an MQTT relay into openHAB 3 using the UI and processing some data from that relay using a Transformation Service - it may help you. If you’re confident that your Broker Thing is setup correctly you could start from step 4.