Hi, thank you. I didn't know about this plugin. The working model was actually not done by me, the messed up instead was done by me using onshape so I was able to fix it. The problem was that on onShape I had two parts and when I exported them I tought they would be automatically merged into one part, instead I had to do a boolean union and then export.
Just to know, in this case I was able to fix since I had the original project, but if I had only an stl downloaded from somewhere with this problem, then what are the steps to fix it?