Related portfolios are “connected” via tags. In order for a portfolio to show as “related”, it should share a common tag with the portfolio that the user is currently viewing at the front-end. Try adding the same tag for both portfolios, and it should work correctly.
Related Portfolio Settings at the Customizer
Of course, double check that the related portfolio checkbox is checked under
Appearance > Customizer > Portfolio, like the screenshot below shows. You can choose to show or hide the whole section, and change the title to whatever you wish.