People of Rep of China was not the founder of UN, so you should not say You Return to UN, you may say Under One China policy, People of Rep of China replace the REPUCLIC OF CHINA (Taiwan), actually, REPUCLIC OF CHINA (Taiwan) was the original founder of UN
It was Republic of China, not Republic of China (Taiwan). Republic of China (Taiwan) was not and is not a country. It does not legally mean anything. When the UN was founded, Republic of China included mainland China and Taiwan. People’s Republic of China replaced Republic of China in 1949.