I looked this stuff up before asking it here and simply couldn’t find an answer… The problem is that when I download an assetbundle from the internet, the mesh collider doesn’t work. I literally need mesh colliders for this project and my character keeps falling through the ground 
bruh i fixed it 
public class AssignMeshCollider : MonoBehaviour
{
void Start()
{
GetComponent<MeshCollider>().sharedMesh = GetComponent<MeshFilter>().mesh;
}
}