Mesh Collider dosen't work

Hi all, I imported a dice modal into my scene,then added the rigidbody component and mesh collider component . but when I was testing , the dice passed through the floor.(the floor object had a rigidbody and a box collider).

please help,why the mesh collider didn't work?

I found these in unity manual:

1: Non-convex Mesh Colliders are only supported on GameObjects without a rigidbody. If you want to use a Mesh Collider on a rigidbody, it needs to be marked as Convex.

2: Convex If enabled, this Mesh Collider will collide with other Mesh Colliders. Convex Mesh Colliders are limited to 255 triangles.

Dose it mean if I want the mesh collider and the rigidbody works at the same gameobject, the mesh have to be less than 255 triangles?