Device Width from the device info column is based on the builder window width while inside the builder. Probably best to preview the app in another window and reduce the size of that window to mobile size. I believe @Darren_Murphy has pointed out a few times that device width is not dynamic, so you will have to make sure the window is the correct size and then refresh the browser window to reload the app.
There is a CSS alternative that is dynamic, but I haven’t explored it personally.