Will I be able to use in Singapore?

The USA version of Apple Watch uses cellular frequencies which are only compatible with the North America cellular region, not available in Singapore. You need the Rest of World model for Singapore, see https://www.apple.com/watch/cellular/ for details of current network compatibility and regional model numbers.

Unfortunately there is is no way to modify your USA watch. However all other functions except cellular will work when paired and connected to your iPhone so it is not a total loss.

I am indeed correct, because the hardware supplied in USA is completely incompatible with Singapore cellular networks. The hardware model number is even different for the two regions, and there is no modification or software patch to change from one regional version to the other.

Call your carrier to help you set up. You could call Apple support but I suspect they would direct you to your carrier. Ta

Sorry this is not correct Singapore is on the list of approved carriers. He still needs to contact his service provider. Ta

May bad! I read as one long list! Doh. Ta

https://www.apple.com/uk/watch/cellular/#table1