A diplomat is someone appointed by a state to conduct a dialogue with one or more other states or international organizations. The main functions of a diplomat are to: represent and protect the interests and nationals of the sending state, beginning, and facilitation of strategic agreements, treaties, promotion of information, trade and commerce, technology, and friendly relations.
One of the main roles of the president is to be the chief diplomat. That being said, Trump is a diplomat. For example, Trump and Israeli prime minister have similar worldviews and in public, seem genuinely excited to be working together. Trump seems genuinely interested in brokering peace within the Israeli-Palestine conflict. Trump seems to be making his own rules as a diplomat.