Yes, there is cellular service in Turkey. Not sure where you got the idea that there isnt.
His phone did not have roaming so he had no service.
Relevant part:
UPDATE: TechCrunchs Zack Whittaker on Thursday posted the enhanced photo below of Khashoggi wearing an Apple Watch Series 3 in Istanbul last May. But according to Whittakers report, the third-generation Apple Watch does not support cellular connections in Turkey, effectively ruling out any chance that his data synced either with his iPhone outside or Apples servers.