As you were talking about impassable terrain, I'll repeat what I've said before:

It seems that one impassable square is buildable (from adjacent), but two ore more are not. Once an impassable square has been roaded, you can cross over to the square behind it if it's not impassable, and build the other way from there.
If there are two impassable squares you can build roads to both from either side, but not one between. Yet somehow you can in fact traverse it.
I couldn't connect the two impassable tiles but I could go over a single one by building from both ends:
https://i.imgur.com/Le6za78.png
https://i.imgur.com/fiGZUmd.png

I see.
I found a mod, Kiame Vivacity's Impassable Map Maker. That fixed the issue by making the impassable passable(the mountains at least. I got the biomes! Mod set with a 3rd party oceans add-on for water).