Arduino stepper motor - too fast

Hi, I am using a stepper motor 17HS4401 and a driver DM512T.

Right now, I can't go higher than 300 because then, the motor does not work anymore. With a setting of 300, I have a speed of ~ 200 rpm.

Driver: https://www.omc-stepperonline.com/download/DM542T.pdf
Motor: https://www.datasheet4u.com/datasheet-pdf/MotionKing/17HS4401/pdf.php?id=928661


How can I reduce the speed without using a gearmotor?

Settings right now on the driver:

1 OFF
2 ON
3 ON
4 ON

5 OFF
6 ON
7 ON
8 ON

Code:


Drehzahl=300;
digitalWrite(SchraubeEna,LOW);

digitalWrite(SchraubeDir,HIGH);
for(Anzahl = 0; 0 < 50; Anzahl++)
{
digitalWrite(SchraubePuls,HIGH);
delayMicroseconds(Drehzahl);
digitalWrite(SchraubePuls,LOW);
delayMicroseconds(Drehzahl);
}