How to make your own Collider for the generated map?

Question

Unity

Help.
I generated a map from Meshes.
I want to add Collider to this map.
I used Mesh Collider, passed Mech cards to it.
But it turns out not under the irregularities of the map.
He builds it inaccurately.
Builds a Collider between the highest point and the lowest.
And the rest of the terrain does not build.
What to do.
Tell.