Node-Red(mqtt) <-> Thingsboard

Hi everyone,
Is someone in this community also familiar with thingsboard. I have problem that i can't solve.
I post the question here because unfortunately thinkboard has no community like this.

Node-red is connected to thingsboard over mqtt. The mqtt Node shows the status "connected". On the TB Dashboard I use a "Knob Control" control-widget and I can send a value to Node-Red. But this widget ever shows a "Device is offline" status. Is anyone here have an idea how to get my device online?

Thank you!