“Trump” would be a masculine noun (der Trump; the def. article can be used with names colloquially); also, while it’s a derivation from “Drumpf”, if we take it as an English noun “trump”, it is literally translated as “[der] Trumpf”. In both cases the declension is strong. Weak declension is pretty much an exception in German for masculine nouns and “en” appears, as a rule, only with weak declension (there are very few exceptions to this, e.g. Sternenzelt). So it would be Trumpführer or Trump-Führer.