Calendar not showing when installing from apk

I am showing a calendar when the user clicks on a field, when I am building and exporting my app directly from unity to my android device the calendar is showing. The problem is that if I export my apk and install it, the calendar is like it doesn't exist, it isn't showing at all. I use the Material design toolkit. Does anyone know what the problem might be? This is the code I use to make it show:

 DialogManager.ShowDatePicker(now.Year, now.Month, Random.Range(1, 30), (System.DateTime date) =>
 {}, MaterialColor.Random500());

I tried putting the prefab of the calendar in the resources folder and instantiate it like that inside the ShowDatePicker:

  GameObject picker = Resources.Load<GameObject>("DialogDatePicker");

   if(picker!= null)
             {
                 Instantiate<GameObject>(picker);
             }
             else
             {
                 Debug.Log("prefab not found");
             }

but it seems that the prefab cannot be found.

Any advice for a possible fix would be really useful to me :)