Player model physics doesn't seem to work properly. Gravity issues maybe?

So I have a player (robot) that is supposed to climb a hill as shown in the pictures.

The robot (player) is the parent gameObject to 4 children (Head, Body, Arms, Legs).

The parent has a rigidbody with gravity on and isKinetic is set to false during runtime.

Each child has its own collider (box, sphere, mesh etc.). I haven't assigned any physics material to any of them.

Collision detection works, so the robot doesn't go through the hill but it doesn't stay on the ground either like the pictures below

As you can see the robot is not moving in an inclined position (first picture) and it is supposed to fall on the ground (second picture).

Please ignore the background as this should be running via AR so I am using my webcam and an additional camera in the scene for testing purposes.

Any help regarding the problem would be appreciated.