Dhl
I send jiffy bags to 40ft containers worldwide for an engineering company, over the years dhl and TNT have lost items,
you need to do is ask dhl to carry out a warehouse search, things have been found in the past miss sorted in a corner of a depot. If after 48 they haven't found it they can deam it lost and you can post a claim with dhl. Be warned due to the Warsaw convention you will not get the value of the frame just a couple of gbp per kg.
Was the address marked on the side of the box or was it In a document wallet? Could this of become detached from the box?
Good luck