Can't reference my script in an imported script

Hi all!

I'm sure this is a dumb question with a simple answer, but I'm having trouble accessing my own scripts from an imported script and I'm not sure why. My scripts are refered to correctly from other parts of the code, but I seem to be missing something in this one. The code I am referring is a simple monobehaviour with a static instance:

 using BansheeGz.BGDatabase;
 using Michsky.UI.Hexart;
 using System.Collections;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.Video;
 
 public class V4_SettingsManager : MonoBehaviour
 {
     /********************************************************
      * This class allows us to hold and save player settings
      * *****************************************************/
     public static V4_SettingsManager instance;
 
 
     private float masterVolume { get; set; }
     private float musicVolume { get; set; }
     private float sfxVolume { get; set; }
 

     public Slider slider_masterVol, slider_musicVol, slider_sfxVol, slider_missionVol, slider_UIVol, slider_StingerVol;
     
     public HorizontalSelector languageSelector;
 
     public delegate void SettingsChanged();
     public static event SettingsChanged OnSettingsChanged;

While the code I want to refer it in is a simple UIElement from a plugin (I'm sharing only part of the code here to not step on any toes):

 using UnityEngine.UI;
 using UnityEngine.Events;
 using NaughtyAttributes;
 using BansheeGz.BGDatabase;
 
 namespace Michsky.UI.Hexart
 {
     public class HorizontalSelector : MonoBehaviour
     {
 
         public bool useLocalization;
         [ShowIf("useLocalization")]
         public string localizationSheet;
 
         private Text label;
         private Text labeHelper;
         private Animator selectorAnimator;
 
         [Header("SETTINGS")]
         private int index = 0;
         public int defaultIndex = 0;
 
         [Header("ELEMENTS")]
         public List<string> elements = new List<string>();
 
         [Header("EVENT")]
         public UnityEvent onValueChanged;
 
         void Start()
         { 
         }
 
         void TranslateElements()
         {
         }
 
         public void PreviousClick()
         {
         }
 
         public void ForwardClick()
         {
         }
 }

Concretley I want to reference the delegate of the settingsManager in the Horizontal Selector script, but it can't find it. Does anyone know what I'm missing?

In advance, thanks. Rafa