Hi,
I can’t access your project right now, but from your description it feels like you are running a CHTv2 study with solids and a surrounding flow region.
If you choose to add a flow region around the cell, then the convection between the cell and the surrounding fluid is being solved (since you explicitly added the flow region). For that reason you won’t be able to apply an h value. In this case, the boundary condition configuration would be similar to the one from this tutorial.
If you do not add a flow region around the cell, then you would be able to assign a h value to the external cell walls. This would be a wall boundary condition, and not natural convection. See this tutorial for reference.
Cheers