Can't separate cell in Matrix

I use matrix, which contain united cells. Like on added picture from example.
And a can't split united cell to small cells. I can't find any option which can help with this.
For example, changing "RowSpan" set effect only in design mode, not in preview.

Leave a Comment

Rich Text Editor. To edit a paragraph's style, hit tab to get to the paragraph menu. From there you will be able to pick one style. Nothing defaults to paragraph. An inline formatting menu will show up when you select text. Hit tab to get into that menu. Some elements, such as rich link embeds, images, loading indicators, and error messages may get inserted into the editor. You may navigate to these using the arrow keys inside of the editor and delete them with the delete or backspace key.