I’m travelling from Gent to Erzurum. I tried booking tickets through the link you give (EYBIS), but they seem not to give any tickets after 17 July, is this normal?
TCDD started to open seats for reservations in last 15 days for high speed trains.
For main line trains, reservation 30 days prior to travel time is still possible.
Please also note that EYBIS does not display trains with no seat available.