For anyone still looking for help with this, I wrote an article/tutorial on how to build Previous and Next post navigation around the Relationship field.
You can check it out below:
https://thatwebdude.com/tutorial/how-to-create-previous-and-next-posts-with-advanced-custom-fields/
This builds around what @sytheveenje was trying to accomplish, but with a little more structure and organization.
I hope it’s helpful!
