Finally got it to work as a US resident. I went on mobile (iPhone) > turned off wifi (use cellular) > turned off VPN/iCloud Private Relay, signed into ChatGPT via a different gmail account than I used with the service previously > set up payment via Apple Pay.
I tried entering the card number in manually and using Link service, on mac/pc/mobile, but not luck. This combination above finally worked for me. Good luck to anyone struggling with this!