First of all welcome to the guild and I am sorry you are having trouble with the rotary knob right off the bat.
I did not have this issue with my GMMK Pro when I had it. One thing you could try doing is a factory reset of the keyboard by pressing and holding down FN + ESC then F1 + F3 + F5 for 10 seconds and see if that fixes the issue, otherwise iirc you should be able to try and reprogram the knob and set it to volume up and down with the rotation if it is still not working. If both those options do not work then I would contact Glorious Customer Support and see if they have a fix to the issue.
Again I am sorry to hear you are having this issue right off the bat and I hope that either one of these solutions works for you.