okay. I finally tried again after a year to fix the navigation issues on nafcom.eu, worked out a solution and emailed it to the theme support guys, who helped me back in 2017 to fix one problem but then ignored me when I re-approached them in 2018 🙁
So here is how it’s done:
1) First set post page as home page: https://stackoverflow.com/questions/16508229/wordpress-how-to-set-my-posts-page-as-home-page
2) Then add your old “home page” to the menu at Appearance -> Menus (e.g. in my case I renamed it to “About” before I did that)
3) Then add a custom link as described in the youtube video here: https://www.youtube.com/watch?v=BlcpNMhka3M and post it to the home page
4) Trash the old blog page to avoid people who accessed nafcom.eu/blog in the past are not confronted by a white page but instead get re-directed to an actual blog post which is next closest to this!