This Friday, 9th February 2018 , will be the launch of Apple Watch Series 3 (GPS + Cellular) in Singapore.
Singtel is the only telco in Singapore to support the cellular function on the Apple Watch.
It is called NumberShare . It costs S$6.90 per month on a 12-month contract. The first three months are free, and that works out to be about S$5.20 per month.
To signup for NumberShare, you need to do it using the Watch app on your iPhone. It can’t be done on Singtel’s end.
Ensure that your Singtel carrier setting is version 31.1. You can check it by going to Settings > General > About.
Once you have verified that, open up the Watch app and go to Mobile Data.
Tap on Setup Mobile Data. After tapping on it, give it a few seconds for it to respond as it will be loading a webpage. If your Setup Mobile Data is greyed out, that means that you did not update your carrier settings.
Once your setup is completed, you will receive an SMS from Singtel to confirm your NumberShare subscription.
Next, open up your Watch app, then go to General > About.
Doing this, Apple will push a carrier update to your watch (similar to the first step). Your watch carrier settings should be version 3.2.
And you are done.
I activated NumberShare on Saturday, 3rd February 2018. It seems that Apple toggled the switch early.
My Apple Watch is from Japan, and yes, it works in Singapore. It is the same model number as those being sold in Singapore. If you intend to get an Apple Watch in Japan, do read my post  on where to get it.
If you are on cellular on Apple Watch (as shown by the four green dots above), the battery can only last for four hours .
Apple Watch will only switch to cellular as a last resort. It will try to connect to your iPhone first, followed by your saved wi-fi networks and lastly, cellular.