Complex sentence: With their help (Czekanowski is considered his mentor) he joined the Russian Geographical Society, got a job at a local museum and took part in several expeditions, gaining experience and prominence.
Split into simpler sentences: With their help, Czekanowski is considered his mentor, he was able to join the Russian Geographical Society. He secured a job at a local museum and took part in several expeditions, gaining both experience and prominence.