Paper claims that there is no solution for a problem I solved. Should I approach authors or editor first if I want that information to be included?
The right thing to do is always to assume the authors of that paper simply were not aware of your work, thus in good faith. Depending on how many (thousands?) paper get published each year in your field, that may be the most likely explanation. Then, contact the authors kindly pointing out your own paper and explaining shortly your conclusions. If they don't take your work/yourself seriously, then write the editor.