I consulted my friends through Whatsapp, many of whom are experienced engineers. Perhaps they are just old sign boards? The fact they have a silvery finish and are fenced in with barbed wire make that unlikely. Were they antennas? There did not seem to be cables connected. So probably not.
Our conclusion is that that they are passive microwave reflectors. They are probably used by TVB (Television Broadcasting Company) to relay TV signals between their production facilities in Tseung Kwan O (將軍澳) to the antennas on Tse Wan Shan (慈雲山) - because there is no direct line-of-sight between them.
Is that what it is?