I have used that train 2-3 times. Usually tickets went on sale at 2-3 am in the morning (CET), but that was 2-3 years ago. Didn’t check anymore recently.

if you are using interrail there you may exchange the ticket for a pass supplement only at the ticket office. Money will be refunded to your creditcard within 1-2 days.

Thanks for advice. I had a look tonight for Erzincwn-Ankara, they went live at 00:20BST on the app and immediately there were zero couchettes available - it looked like it would be impossible to book a sleeper at all. 
 

does anyone know or any agencies which sell couchettes for the Dogu express? I’m happy to sleep on a Pullman, but my partner has insomnia and will really suffer on a 17hr train on a seat!

A good thing is to try approximately 10 days before again when many people start giving back their tickets…. When I had a seat only I met several people who got their tickets approximately 10 days before for the couchette.