I found level difference between level 19 and level 20 at the top left of the map. But after I reach two pipes (probably those that are mentioned in the stuck achievement) i can't pass them. I've tried to wiggle inside of them, but it dowsn't seem to work. The rest of the map is the same. Am I missing something? Is there another map part? Or maybe I should complete secret levels first...

You can open two main menus by following these steps: 1) Click on "multiplayer" sticky note. 2) Open the settings. 3) Press key on keyboard. Here's a video showing how to do it.