Imported Mesh falls through floor. Cannot use RigidBodies or Mesh Colliders

My 3D mesh falls through the floor, even when I add rigidbody and/or mesh collider components to it! The terrain itself is solid; when I add cubes, they recognize the terrain.

I’ve tried to add the mesh colliders, but the green outline for it is shaped sideways! Nothing from the Unity help sections or manual was able to help me!

By any chance does your “floor/terrain” a mesh collider? If so you may be having this issue detailed in the reference:

There are some limitations when using the Mesh Collider. Usually, two Mesh Colliders cannot collide with each other. All Mesh Colliders can collide with any primitive Collider. If your mesh is marked as Convex, then it can collide with other Mesh Colliders.

See Mesh Coliders.