Welcome to Mobileread
I'm not sure what you mean by you transferred the ebooks according to kobo instructions
So I'm not sure I can help you re that aspect of your issues.
I can however introduce you to Calibre, which is an ebook manager that is enormously popular and well managed by people who contribute to mobileread.
Here is the link:
https://calibre-ebook.com/
I use calibre to side load my ebooks to my various kobo devices and have done so for well over a decade.