In the image below, notice the Frame with the red dot, this the the Frame that immediately wraps your columns. Apply Device Stack to this Frame. Please note, you only need to add Device Stack once, you do not need to use the class in the columns themselves. For your convenience, in the base MOBLE layouts, an extra Frame wraps all columns by default, this is intended for such tools.
Single is the default setting. E.g. if your page has 3 columns, they will be stacked each column on top of each other, with left on top.
Double is a popular control and stacks your columns in double. E.g. if your page has 3 columns, the first two with appear in 2 columns on mobile, with the third below.
Fixed is used to fix all all columns to the same row, effectively, disabling the stacking action. For example, this might be useful where you have a set of icons that you'd like to line up next to each other.
Use the device tools in the top bar to preview your website on each device.