With my Clara HD, I can upload with calibre but what I do more often is to convert epub to kobo specific one Kepubify. The reader can read regular epub but you don’t have book progression, meta data etc the same as with the kepub.

Then put the result on a local web server (even possible on android if you’re on the go). I then use the built-in kobo web browser (in beta menu) to browse and download the book.