Hi Marcus,
I suggest you parent your coin in an empty GameObject so you can move it around indenpendently from it's rotation behaviour.
Using DoTween Pro, you can set a infinite loop settings for a relative local Y rotation.
See my attached image for the hierarchy and the DoTweenPro component options.
Otherwise, you can add a component to your CoinRotation object and rotate it by code:
transform.Rotate(Vector3.right, 1.5f);