Pages: rotate existing, wide table by 90 degrees so that the rows are now vertical possible? (equations in several cells)
In order to rotate a table, the document must be changed to a Layout type of document.  Also, be aware that this is a bit of a hack, your mileage may vary.
Note: It may be best to start with a new document or a copy of your current document to be safe.
Select Document to reveal the Document sidebar.

Next, untick the Document body checkbox.  You will see a dialog that asks you to confirm the change to your document.
Select Convert.
Add a new Table and edit its contents first.  You cannot edit the table once it has been grouped. ‍♂️
Add a shape (hint: put the shape behind the table to hide it)
From the Format sidebar, choose arrange.
Select both the Table and the shape.  Shift+click or Drag selection.
Select Group.
Now Rotate the table.