Actually, not only was the ROC recognized by the UN but it was a founding member of the UN.
TAIWAN’S TOUGH HISTORY WITH THE UNITED NATIONS
Just a snippet from the link to not derail.....
"
Taiwan has had a long and dramatic relationship with the United Nations, from being — as the Republic of China (ROC) — one of the founding members of the UN in 1945 to losing its seat at the National Security Council to the People’s Republic of China (PRC) in 1971. The main issue with Taiwan’s membership in the UN is based on two seemingly insuperable concerns: a concept of membership expressed in the Charter of the United Nations, the foundational document on the organization, and the so-called “one China” policy officially accepted by the UN.
The Charter of the United Nations states that “the membership in the United Nations is open to all peace-loving States that accept the obligations contained in the United Nations Charter and, in the judgment of the Organization, are able to carry out these obligations.” The operative word is “States,” since membership as the ultimate form of participation is indissolubly related to sovereignty —
de jure sovereignty, and not
de facto, as is presently the case for Taiwan.
De facto Taiwan is and has been a sovereign Western-style liberal democracy for decades.
Yet it is cursed by the “one China” policy, which claims that “the Government of the People’s Republic of China is the sole legal government of China and Taiwan is its inalienable part.” Those who want to have diplomatic relations with the PRC must break ties with the ROC."