Most eLearning courses provide a sidebar with access to chapters.
SmartPhones and some tablets do not have enough screen real estate to provide horizontal room for both navigation and content.
If you have less then two or three chapters in a course you can provide navigation as a bar along the bottom of a screen.
If you have more then two or three chapters you should consider integrating the navigation with the entry screen and using home or map for global navigation.
Page numbering is very important with mobile apps. Learners do not like to blindly take a course. They like to know where they are in a course.