It seems that they don't work in the overworld, or is there something I'm missing?

This isn't explained very well in the game. But in the room where you found that key, the example portal that you took the key out of contains functional modifications. Modify your portal to match that one.

https://github.com/AllenSeitz/DimDungeons/wiki/Advice-About-Advanced-Dungeons

Oh, That's what those weird "fence" looking blocks are! Ok, thanks!