Value set in function doesn't update the glabal value

So I'm making the space shooter tutorial game and I was mockin around a bit to see if I could create a system where the asteroids would coma faster at you after a set amount of waves, however.. I'm now completely stuck on a piece of code that just doesn't want to work...

So I made a few variables that count down how many waves have appeared and when a certain limit has been reached I want it to update the speed which I did like so

 public class GameController : MonoBehaviour {
     // bunch of other variables
     public Mover spd 
     public int speedRounds
     private int speedCount
     
     IEnumerator SpawnWaves (){
             yield return new WaitForSeconds (startWait);
             while (true) {
                 if (speedCount < speedRounds) {
                     speedCount++;
                 }
                 if (speedCount == speedRounds) {
                     speedCount = 0;
                     spd.Faster ();
                 }
     
                 //code for making the asteroids spawn

following up with the following mover script where Faster(); is calling to

 public class Mover : MonoBehaviour {
     private Rigidbody rb;
     public float speed;
     public float maxSpeed;
 
     private float speedCalc;
     private float maxSpeedCalc;
     float speedMin;
     float maxSpeedMin;
 
     void Start () {
         Speed ();
         rb = GetComponent<Rigidbody> ();
         rb.velocity = transform.forward*speedCalc;
         //Debug.Log (speedCalc+ "," + speed+ ", " + speedMin);
     }
 
     public void Faster () {
         if (maxSpeedCalc > 0) {
             maxSpeedMin++;
             speedMin++;
             Speed ();
         } 
         else {
             return;
         }
     }
 
     void Speed (){
         maxSpeedCalc = maxSpeed - maxSpeedMin;
         speedCalc = speed - speedMin;
     }
 
     public void Reset (){
         speedMin = 0;
         maxSpeedMin = 0;
     }
 }

The problem is that, for some reason that I can't understand, it will update speedMin inside the void Faster () but nowhere else. Whenever I debug inside Faster () it will steadily count up the speedMin variable (which is also why I needed to put in the Reset () function which is called in MasterController's Start () so that they will reset when the game begins). However, when I debug anywhere else in the code it tells me that speedMin is still 0...

I'm pretty new to Unity (hence the tutorial) but I have some basic programming knowledge so I normally have a pretty good idea of what I can and cannot do but this just baffles me..

If anyone can help it would be most appreciated!