To use a Singtel eSIM in Singapore, your device must first be carrier-unlocked.
For iPhones: Go to Settings > General > About. If “No SIM restrictions” appears next to Network Provider, your iPhone is unlocked.
For Android phones: Go to phone Settings > Connections > Mobile networks > Network operators > Turn off Automatic option. If you can view other carrier available networks, your Android phone is likely not carrier-locked.