Creating a spherical world

Hi. Unfortunatly non-convex mesh colliders don't work at all in Unity 5 without a rigidbody assigned ("Non convex mesh collider with non kinematic Rigidbody is no longer supported in Unity 5"). But if I add a rigidbody I have to make it kinematic and then collision detection is no longer working for me. I made a low poly shape of the earth with about 12 500 tris and I really have to collide properly with it and since the earth is really large (I made it half as big as real size actually, where 1 unit in Unity equals 1 metre) and thus giving it a sphere collider would be painfull as you could walk through mountains like the $$anonymous$$ount Everest and you also could not go into water.

EDIT: Very weird. I found this link from Unity that says that collision detection should work with a kinematic rigidbody collider and a rigidbody collider. https://docs.unity3d.com/$$anonymous$$anual/CollidersOverview.html That is not true for me. Since my earth is still too large for Unity somehow (even though in blender it had 12 500 faces / tris so less than 2 ^ 16 [Unity Limit till it splits into multiple meshes]) I have two meshes that have a parent. Now i have a test object that falls towards the earth by the earth's gravity. It only does this if it's not on the ground allready. $$anonymous$$y scripts work fine with a simple sphere as the earth, so with a sphere collider. That's what my system looks like:

 Test Cube (Falling towards earth, though OnCollisionEnter is not firing, has Rigidbody)
 Earth
     Earth $$anonymous$$esh1 (Contains a non convex mesh collider and a kinematic rigidbody)
     Earth $$anonymous$$esh2 (Contains the same as mesh 1)

... so there is a kinematic rigidbody (the earth meshes) colliding with a non kinematic rigidbody (the test cube). Why is OnCollisionEnter not firing then? I don't get it...