Send the accepted contents and your receiving count, then ask for the missing items' package mark and actual collection record, because the arm's receipt cannot trace another package.
Ask them to identify that carton against your order and say who physically holds it now; a photograph of a controller box doesn't give the forwarder much to search.
Have they given a concrete cable supply answer, Theo, and corrected the order status so the next sales contact does not send you back to the arm delivery again?
And give receiving that exact missing-item list for the eventual arrival; finding the controller carton should not let the arm cable disappear from the check again.