That is because you have not customized those breakpoints yet.
After you click on one of those tabs, go ahead and select one of your elements and change it so it looks the way you want it at that size. Webflow creates CSS that will vary depending on the size of the screen.
For instance, if I click on the mobile-portrait, the Nav bar will not show correctly. If I remove any of the images, i.e. the US Navy, etc. I removes it on the desktop as well.
Ah. Removing an element is different from styling it. If you want something to only show up on desktop, then what you do is select it on desktop, then switch to tablet and click the display:none button, and that will hide it on anything smaller than desktop size.
The navbar is an example of something that you need to style separately on each breakpoint. I haven’t looked too deeply into your project, but my bet is not that the navbar is disappearing, but that it is being covered by something else when the screen is too narrow.
Even though I’ve unselected items on the navbar(i.e. all of the symbols for USNavy, US Marines), the links(i.e. Home, About, Photos, etc) still do not show up on the mobile landscape or portrait views.