Just an update: Back when the Series 3 with LTE was launched, only 11.2.5 (and 11.2.6 now) is supported together with watchOS 4.2.2 (and 4.2.3 now) supports NumberShare.
And those who were on iOS 11.3 Beta 2 were out of luck and downgrading back to 11.2.5 meant losing most of the data.
Waited and with iOS 11.3 Beta 3 and watchOS 4.3 Beta 3, I have managed to successfully activate NumberShare. (Though, it is not recommended to update to 11.3 if you’re already not on any of the betas)
The activation process may take longer than 5-10 mins but to just leave it be, which I’ve had itchy fingers from my first activation and I removed the carrier plan via the Apple Watch app cause it took longer than 15 mins, which screwed up the process.
Removed the NumberShare subscription via the Singtel App and waited for 24 hours so as to get the number ‘removed’ from the server.
After which, I’ve got through the same process again and let it do it stuff. Soon after, managed to get it working and activated.