In July 1940, Roller, having narrowly escaped re-arrest by the Romanian authorities, left for Bessarabia, which had been recently occupied by the Soviets. He was for a while at Reni, in the Ukrainian SSR. Roller subsequently moved to the Moldavian SSR, at Chișinău, where he began working for the city's Tobacco Plant. In June 1941, just days before the start of Operation Barbarossa, he sent his résumé to Boris Stefanov, asking to be considered for membership in the Communist Party of the Soviet Union. Drawn into the PCdR group in Moscow, Roller continued working under Pauker, who had also joined the Romanian exile community. He also attended the History faculty of Moscow State University. Little is known about Roller's activity in early 1944, when the change of fortunes on the Eastern Front signaled a Soviet victory over the Axis Powers. Reputedly, he stopped paying his PCdR membership fee, which may indicate that he was busy with party work, and prepared for a career in communized Romania. After spending some time at Institute No. 205 (formerly a Comintern school), in December 1944 he was the only Romanian native teaching prisoners of war at Krasnogorsk's Central Antifascist School. While here, he suggested employing Alexandru Bârlădeanu and Haia Grinberg to assist him with specialized classes.