In either case wearing and detaching a visor that was mounted on the helmet that became badly damaged, or finding a badly damaged visor and trying to attach it to the assault helmet won't work.

Happened to me and a friend- I was wearing it when it became badly damaged during a fight... I detached it and then went to try and reattach it and could not.

My friend found a visor that was listed as badly damaged and tried to attach it but couldn't. In both cases the common denominator was that the visor was badly damaged and the helmet was worn.