How to determine when player (cylinder) has moved above a plane in unity 3D?

Question

I am making a delivery game and temporarily designed my game so that the player who is a cylinder at the moment has to locate the delivery location which is a flat plane and move onto it in order to deliver the objects.

I have tried adding a 2d box collider to the plane to detect when the player collides with the plane but the collider only works on the x,y axis and i am not able to rotate it 90 degrees.

So my question is what is the best way to determine when the player has moved above the plane?

Here is a picture to demonstrate what i mean:

Before moving onto plane

How to i detect this:

Player is on top of plane