If I understand well you are trying to switch from a fullscreen app to another app aren't you? If it is the case, the Alt+tab would not help you because when an app goes fullscreen the os automatically moves it to another space but alt+tab is a keyboard shrotcut to swtitch applications on the current space only.
In this case what you want to achieve is to switch the space, and for this reason you have to use Super+Tab keyboard shortcut. Another options are Super+a, Super+s and Super+w
Super = windows like key on your windows keyboard or Cmd if you are on a mac. By default if you press only the Super key it triggers a command that shows available keyboard shortcuts.