Turning a 2 column to a single column layout

This is a two part question. The first part is that I want my 2 column layout to turn into a single sectioned page, when hiding the 2nd column on mobile, everything centres which is what I want - however I can’t make this happen with my tablet preview.

Here is my site Read-Only:

If you refer to tablet landscape it will not work. Tablet landscape is presentet as a normal web page

I’m not sure I understand your answer. If I have made one of the columns is hidden in the tablet landscape view - wouldn’t it mean that I have a full-width sectioned website?