Apple News Unavailable. But my account was registered in US

You say it was registered. Does that mean it is still registered in the USA? If not then by changing country settings it is no longer registered in the USA and the only items available to your are the ones available in your new country.

The sections below are copied from the USA Apple Media Services Terms and Conditions document but I suspect they are very similar for all versions of this document --> https://www.apple.com/legal/internet-services/itunes/us/terms.html - Last Updated: September 20, 2021

"Our Services are available for your use in your country or territory of residence (“Home Country”). By creating an account for use of the Services in a particular country or territory you are specifying it as your Home Country."

"Terms found in this Agreement that relate to Services, Content types, features or functionality not available in your Home Country are not applicable to you unless and until they become available to you. To see the Content types available to you in your Home Country, go to the Services or visit https://support.apple.com/en-us/HT204411. Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."

Check the region set for your iPhone in Settings->General->Language & Region. Make sure its set to the U.S. or another country that does support Apple News.

Link-> Availability of Apple Media Services - Apple Support

Read what I posted and highlighted. It is possible that you cannot get Apple News, USA in Singapore even when you are signed in with USA settings. Apple could be blocking the feed or the Singapore government could be, or maybe it is some other reason.

In which case Limnos response would be correct. Apple News is not available in Singapore, so it may not be possible to offer it there when traveling. I doubt its Apple actively blocking it, but it could be a regional or government blockage.

If no settings have been changed, and all is correct, then it's simply not available there and you cannot use it while you are traveling there.