Poles had pretty good mathemathicians at the time https://en.wikipedia.org/wiki/Polish_School_of_Mathematics

Germany was considered a threat (Piłsudski even proposed preventive invasion of Germany in 1935 to France and UK, which declined).

They also had some luck guessing the rotors arrangement in early military versions, and noticed some regularities caused by German operators being lazy (repeating common phrases etc). That gave them something to start with.