How do I modify STL objects?

I’m stumped on modifying an STL object I imported into Shapr3D. I imported the STL fine, and I want to cut-away part of the model. I drew an ellipse, I extruded it so that its body intersected with the STL part, I tried to use the Subtract tool but nothing happened. Is this not possible? I have images of what it looks like at Index of /shapr3d of what I did.

I’m doing this to put a notch in an existing phone/tablet stand to allow touches to the bottom center of the screen.

Shapr3D has very limited support for STLs. Basically, they can only be used as bodies with which to perform Boolean operations on other objects. While that may seem useful, unfortunately the resulting body after such an operation is always an STL.