They made life difficult for the people in power, but never ruled on their own. During the '30s they were quite a nuisance, they assassinated 2 or 3 prime ministers, a minister of justice, and the head of the state security services after several governments tried to ban them. The feeling was mutual, the King cracked down on them hard by the end of the decade. By 1939 the original leadership had been eliminated because they were "shot while trying to escape from jail" like Corneliu Codreanu.
The Legion advocated an alliance with Nazi Germany, but the Romanian government was pro-French before the militarization of the Rhineland, and tried to stay neutral after that. German-Romanian relations are a good case study of conflicting priorities from different parts of the government. Hitler and the Nazi party preferred to work with the King or a dictator like Antonescu. The SS, on the other hand, was ideologically sympathetic to the Iron Guard.
Many of the second tier leadership like Horia Sima who survived the crackdowns of the late '30s fled Romania for Germany, where they also received funding and support from the SS and Rosenberg.
The Iron Guard was brought into power in September 1940 as a kind of junior partner, but Antonescu was still the in driver's seat. Fascists promised a revolutionary transformation once they were in office, so being in government but unable to deliver on their message torpedoes a lot of their popular support. Romania's government is like if the Nazi-DNVP coalition from early in '33 had actually lasted with Von Papen and the conservatives still in control.
The Iron Guard attempted to launch a coup in January 1941 by seizing a couple ministries in Bucharest, but the attempt only lasted 3 days. Guardists also launched a brutal pogrom against the Jews of Bucharest to try and portray their enemies as being on the same side as the Jews, a massive negative in the eyes of Romania's antisemitic peasantry.
Antonescu, a mildly antisemitic dictator who promised to ally with Germany and retake Romania's lost territories, was close enough to the fascist promise that most would-be fascist supporters settled for the conservative authoritarian instead and he co-opted much of their platform.