Nothing leaps out. What MQTT message is your device sending?
Your Thing is listening for “temperature”, and that is all. Mine are more complex than that, but that’s the data my devices are sending. If you’ve got the wrong topic, the value won’t update.
If you subscribe with the command line tools, what messages do you see?
What makes you think that MQTTfx needs a more specific topic subscribed to then openhab?
For MQTTfx you use “sensor/temperature” for openhab only “temperature”.
Try updating your channel to search for /sensor/temperature just like you have it in your tool. Maybe it’ll see those values. Mine are all complete topics like that.
Everything looks good in the Things and Items file so changing from string to number was just me guessing.
Have you restarted OH since creating or modifying the mqtt files? If your using OH 2.4 there is a bug/issue that requires this for mqtt to work each time you make a file change. Use the commands below and see if you start getting the sensor value on OH.