I don't know. I'm just letting you know about the Student license limitation.  Are you on the Student license?  Is this a 2-way coupling?

The 2-way coupling I know involves Deformation and Stress on the Structural side.  Fluent doesn't compute stress.

Fluent can compute heat transfer through solids and does transient simulations so why do you need to use a Transient Thermal at all?  Can you do the whole model in Fluent?