I submitted my revised manuscript to a mathematical journal since 12-04-2017. The status is "Waiting for reviewer assignment" since 24-4-2017. What does it mean? Have the reviewers started working on my manuscript yet?.
It typically means that the paper has been received, has been put in the work queue of the editor, but the editor has not assigned reviewers. Or (in one system I know of), that the editor has not assigned the necessary number of reviewers. That could be because the system requires four reviewers, but the editor has only gotten the commitment from three people to review the paper. (Which the editor may deem enough, and so the status will never change until the three reviews have been received.) Or that the editor has asked four people, but only three have accepted. (Which again, the editor may deem sufficient.) Or that the paper has been put in the editor's queue but he has not done anything with it so far.
In other words, the status may mean that the editor is asleep at the wheel, or it may not. You can't draw any inferences just from the status.