dropbox integration is for Forma only, not for Aura. kobo web browser to dropbox may work but I recall reading elsewhere that there are issues with that.
e.g. this from reddit:
https://www.reddit.com/r/kobo/commen...n_web_browser/
"How do you add book using built in web browser and Dropbox?
I created a link for an ePub on Dropbox and navigated to the page using the bulletin web browser on my Kobo Aura but clicking on the Download link for the ePub file doesn't add it to my library. "
try it & see/ study the reddit discussion