如何在调整大小时更改div的位置？

I have a problem, all I want to do is at a certain width (using the @media) I want to change my left div to the right side under the one that is already there. I tried everything, including changing the display, positioning using left margin but due to the parent container it doesn't work. The only thing that I thought about is to make the same left div on the right side and to make the display to none, but I am pretty sure that's not the way to deal with it. I use Bootstrap 4.I have a problem, all I want to do is at a cert