Unity Ads working fine on Unity Editor (pc) but not in Android phone

Question

I have made a banner ad by using Unity Ads. In my pc seems to be working fine, but when I export it to my android phone no ad is shown. Any idea how to solve this? This is the code that I am using: (I did not put the actual app id due to privacy)

public class AdsManager : MonoBehaviour
{
    string appId = "....";

    void Start()
    {
        Advertisement.Initialize(appId);
    }

    void Update()
    {
        ShowBanner();
    }

    void ShowBanner()
    {
        Advertisement.Banner.Show("Banner_Android");
    }
}

I read the documentation, whatched Youtube tutorials, tryied to use a corroutine