Refund declined because broken part is an "accessory"

I ordered an M3 fitness tracker. It was delivered twice without the charging cable. Eventually they managed to deliver it with the cable but after a week the "outer cover" of the fitness tracker fell off and was lost. My exchange / refund was declined because I couldn't provide all the "accessories". I tried explaining that the lost part was not an accessory but part of the product, but my refund was still declined.