TERRAIN AND COLIDER on VRCHAT

Hi all!

I am building a landscape to walk around in VRChat. I am using the terrain tool and it has the Terrain Collider by default.

I think I understand the tools more or less and made a decent beach scene as you can see in the picture.

The problem is, when I test it in vrchat, I can walk most areas, I can even walk downhill but when I try to go uphill or walk in the river, the avatar falls as it happens when you reach the limits of the world.

Any idea why it happens? How can I make the whole floor supersolid? and how can I export the 3D model so I can work the meshes in Blender?

Thank you very much!!