I checked nearly every answer for this, but those were mostly simple errors and mistakes.
My problem is that OnCollisionEnter is not called even when colliding whith other r
Make sure that object you wish to collide with target has non-kinematic rigidbody and mesh collider. My hitter object is a cube and just change its collider to mesh collider
On mesh colider inspector make sure you enable convex. Please see more mesh collider inspector detail here
Now your OnCollisionEnter works. I hope this helps you.