Failing every method for touch point in Android screen

I can't see why on earth this script runs perfectly OK in the Unity Editor but fails in Android tablet. I suspect that the problem is in getting the mouse events because it is not going inside the if block no matter the method I use as condition (see the +++comment+++ in code), probably due to the difference between mouse and finger positioning. Yet I can't figure out the solution. I coded a sound to be played in the if block, so that I know where exactly the problem is.

This is a 2D game where I must draw a line over a predefined path (collider 'area_0' or 'area_1') without getting out of it. So I'm raycasting finger position against the collider.

In Unity Editor works like a charm, once I push it to the tablet it just doesn't.

I adapted the line drawing script some guy published in YouTube tutorial.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Dibujar : MonoBehaviour {
     private Color color;
 
     void Start() {
         color = new Color32(0xFF, 0xED, 0x00, 0xFF);
     }
 
     // Update is called once per frame
     void Update() {
         if (Input.GetMouseButtonDown(0)) {
         //if (Input.GetKeyDown(KeyCode.Mouse0))
             // +++++++ DOES NOT GET IN HERE +++++++
             StartCoroutine(dibujar());
         }
     }
 
     IEnumerator dibujar() {
         LineRenderer linea = new GameObject().AddComponent<LineRenderer>();
         List<Vector3> posiciones = new List<Vector3>();
         bool borrar = false;
 
         linea.startWidth = 0.2f;
         linea.endWidth = 0.2f;
         linea.material = new Material(Shader.Find("Unlit/Color"));
         linea.material.color = color;
         linea.transform.SetParent(transform);
         linea.numCapVertices = 1;
         linea.numCornerVertices = 1;
 
         while (Input.GetMouseButton(0)) {
         //while (Input.GetKey(KeyCode.Mouse0)) {
         //while (Input.touchCount > 0 || Input.touches[0].phase == TouchPhase.Began) {

             posiciones.Add(Camera.main.ScreenToWorldPoint(Input.mousePosition) + Vector3.forward);
             linea.numPositions = posiciones.Count;
             linea.SetPositions(posiciones.ToArray());
 
             Vecto3 rayo = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             Debug.DrawRay(rayo, Vector3.forward * 20);
             RaycastHit2D hit = Physics2D.Raycast(rayo, Vector3.forward, Mathf.Infinity);
             
             if (hit.collider == null) {
                 // if no hit
                 }
                 borrar = true;
                 break; // force while loop to stop drawing
             }
 
             if (hit.collider.gameObject.name == "area_0") {
                 // if hit area_0
             }
 
             if (hit.collider.gameObject.name == "area_1") {
                 // if hit area_1
             }
 
             yield return new WaitForSeconds(0);
         }
 
         if (borrar) {
             // erase line
         }
     }
 }