Hi, if you have access to the original design I will recommend that you export in STEP or IGES format instead since you need to further edit the design. The STL format is more of a final output file format and does not best support design editing.
Workarounds can be to try spliting the design using the subtract Boolean tool: https://www.youtube.com/watch?v=0JVbtFBuCPs&t=1s
or converting the STL file to an STEP file using Freecad so that you can properly edit the design. Here’s how to go about the conversion using FreeCAD:
https://grabcad.com/tutorials/how-to-convert-stl-to-step-using-freecad