Got rejected with limited scientific validity
You write:


  The topic is regarding the development of a numerical code where I compare the results with the theoretically expected ones. Therefore I do not solve a problem but show that the numerical code works,


The question then becomes from the perspective of a reviewer: What is it about the paper that is novel? If I understand the quote above correctly, then (i) the numerical method you use is not new, (ii) there already exists some theoretical analysis of the method, and (iii) the problem you are solving is also already known. So this sounds like you implemented a code that can solve this problem and produced numerical results. But this is not new in itself -- other people have likely done this before, and unless your paper is about the technical details of this implementation and how they differ from how other people have implemented these algorithms, there really isn't anything new in your paper.

As a consequence, it seems entirely reasonable to me that your paper was rejected: Just doing what others have done before is not science.