Select single mesh in actor

I have import a 3d model, in the static mesh viewer I can select each rectangle individually.

But once the static mesh is inserted inside an actor, it is no longer possible for me to do it.

I would like to have the opportunity to move and rotate each rectangle individually inside the actor.
sorry for my ignorance and thanks in advance!

You will need to import the elements separately if you need to manipulate them the way you describe.

So it is not possible to separate the two parts of the static mesh inside the Unreal right?
do I have to separate them externally and re-import them?

They cannot be separated. If the elements are identical, import only 1 and duplicate it.