There are work arounds to doing this. However the problem is the user experience is not preferred. Basically when trying to open up a Notebook on an iPad or iPhone or any mobile device without the ProPlus version of OneNote from a SharePoint or OneDrive App the users is brought to a login screen. The OneNote mobile clients will not allow users to login to their on-premise SharePoint environments. Does anyone know if it is on Microsoft's roadmap to allow for this or it just is what it is?