Hi Joke van der Kraan,
Thanks for contacting Styled Themes Support team.
We are really sorry for the inconvenience, now we don’t have featured to show previous and next post link on single portfolio. But will surely make this feature available on next version.
If you are technically sound on WordPress, you can customize single-portfolio.php template placing on child theme. And you can place the_post_navigation() function inside the query loop. To know more on this please visit here: https://developer.wordpress.org/reference/functions/the_post_navigation/
Note: Please customize the single-portfolio.php template placing it on child theme otherwise your changes on parent/core file will be overwritten by new theme update.
2> You need to manage design for navigation accordingly.
3> And proceed the above procedure, only if you are technically sound on WordPress.
Thanks