Volume control with Suptronics X920 DAC

I’m trying to control the volume with the Suptronics X920 DAC using the socket and REST APIs unsuccessfully. When the volume is at 100 and I try to volume down it will get at 86 and then it will up to 100. When the volume is under 50 and I try absolute volume values or plus/minus commands. It will just show NaN or null values. Any help appreciated. Thanks!