Import .stl file as geometry, not reflecting the geometry correctly

Hello all,

I am trying to import an stl file into MATLAB as the geometry for a heat transfer analysis. But the imported geomotry is not the same as that in auto CAD, where it was created. The geometry created in autoCAD is a big box containing a smaller box, however, in MATLAB geometry there was only one box and saying only one cell.

Is there any methods to fix this? Is this an autoCAD problem or MATLAB problem?

Thank you very much!

Best regards,

SS