There is no standby policy on AFKL, except on full fare tickets. You can change an award ticket by paying a change fee of €45 + an eventual repricing and you have to do this in advance and not show up at the airport hoping to solve this. Airport contractors (or possibly GA agents) serving this flight are not able to handle FB ticket changes.
I am, like you, surprised that the DPS-SIN sector is full in both Y and J. You may also find availability on GA either on their non-stop services or with a connection in CGK.