How can I change my Apple Account region from China to Singapore?

I was on a business trip to shanghai this Mar (first time ever and occur such problem), ever since back, my phone recognizes as China region. I have already done all method that was taught online (AppleID, phone setting, reset location and region setting), region shows Singapore everywhere in phone, but certain apps recognize me as China user.

Eg, there's this app pinduoduo (an online shopping app similar to Shopee) recognize my location as China whereas my sister (who's using same iPhone 16 Pro Max as me, IOS update to latest) download as International user. She's able to make purchase as international buyer but I can't as the phone recognizing my region in China, but my current location is in Singapore.

I'm wondering why and how I can fix it back to normal.

[Re-Titled by Moderator]

Original Title: Region in China despite setting are all set to Singapore