Hello Kararara,
Thank you for using Apple Support Communities!
We understand from your post that you wish to purchase an App Store gift card so you can redeem it to your Apple ID which is currently registered with an American phone number and billing address, but you are currently in Singapore. You would need to purchase a gift card that was in USD not SGD. You would not be able to redeem a gift card purchased at an Apple Store in Singapore and then add it to your Apple ID as the currency would not be the same monetary unit. You may be able to purchase a gift card online in USD and then redeem that gift card. This article has additional information about gift cards which you may find helpful:
What type of gift card do I have? - Apple Support
Best Regards.