Hello,
Please refer to these articles:
http://techwelkin.com/wordpress-split-divide-post-multi-part-pages
http://www.wpbeginner.com/wp-tutorials/how-to-split-wordpress-posts-into-multiple-pages/
http://www.inkthemes.com/easily-split-long-wordpress-posts-in-multiple-pages/
https://pagely.com/blog/2015/03/wordpress-auto-post-pagination/
Regards,
Eva Kemp.