The phrase "diplomatic representative" is correct and usable in written English.
You could use it when referring to someone who is appointed by a country to represent it in foreign relations and policy discussions. For example: "The president appointed a new diplomatic representative to the United Nations."
After the treaty was signed, Max von Brandt became diplomatic representative in Japan – first representing Prussia, and after 1866 representing the North German Confederation, and by 1871 representing the newly established German Empire.
Britain's Foreign Office announced that the Syrian chargé d'affaires, Khaled al-Ayoubi, the top Syrian diplomatic representative in Britain, had resigned because "he is no longer willing to represent a regime that has committed such violent and oppressive acts".
In 1999, just after the Kosovo war, Stewart became Britain's sole diplomatic representative in Montenegro.
Senior Special Operations commanders pledged that their efforts would be coordinated with the senior diplomatic representative in each country.
With the Bolshevik seizure of power in Russia (October 1917), Litvinov was appointed diplomatic representative in London.
Shorten says the Ukrainian diplomatic representative in Australia believes a surface-to-air missile has shot down the plane.
The United States has diplomatic representatives in Taiwan but has formal diplomatic relations only with China.
The extraterritoriality of ambassadors and other diplomatic representatives is likewise of long standing.
Diplomatic representatives from China, Ukraine, Nepal and Russia were also present.
Several countries sent diplomatic representatives to the airport to safeguard Mr. Guaidó's arrival.
Though unable to meet with King, Nehru inquired through diplomatic representatives concerning the possibility of King traveling to India in the future.
