golang fly.io - dial tcp 127.0.0.1:3306

Question

I have a database hosted on a platform called "hostinger", and the database is a mysql database, and my connection to it is like this: package database

import (
    "database/sql"
    "fmt"
    "log"
    "os"

    _ "github.com/go-sql-driver/mysql"
    "github.com/joho/godotenv"
)

func Init() *sql.DB {
    err := godotenv.Load(".env")
    if err != nil {
        log.Println("Erro ao carregar variavel de ambiente", err)

    }
    host := os.Getenv("HOST")
    database := os.Getenv("DATABASE")
    user := os.Getenv("USER")
    password := os.Getenv("PASSWORD")

    connection := fmt.Sprintf(`%s:%s@tcp(%s)/%s`, user, password, host, database)


    db, err := sql.Open("mysql", connection)
    if err != nil {
        log.Println("Erro ao conectar com o banco de dados", err)
    } else {
        log.Println("Sucesso ao realizar conexão com banco de dados")
    }
    return db

}

in my controller I use the connection with mysql like this:

package controllers

import (
    "fmt"
    "log"
    "net/http"

    "github.com/gin-gonic/gin"
    "github.com/guilherm5/database"
    "github.com/guilherm5/models"
)

var DB = database.Init()

func NewUser(c *gin.Context) {
    var newUser models.User
    fmt.Println(DB)
    err := c.ShouldBindJSON(&newUser)
    if err != nil {
        log.Println("Error in read body", err)
        c.JSON(http.StatusBadRequest, gin.H{
            "Error in read body": err.Error(),
        })
        return
    }

    query, err := DB.Prepare(`INSERT INTO usuario (nome, email, senha) VALUES (?,?,?)`)
    if err != nil {
        log.Println("Error in prepare query insert", err)
        c.JSON(http.StatusInternalServerError, gin.H{
            "Error in prepare query insert": err.Error(),
        })
        return
    } else {
        query.Exec(newUser.Nome, newUser.Email, newUser.Senha)
        log.Println("Success")
        c.JSON(http.StatusOK, "Success")
    }

}

on my localhostb everything works as expected, I can insert users smoothly, but when I deploy the application on fly.io and try to send a request, I get the error:

{
    "Error in prepare query insert": "dial tcp 127.0.0.1:3306: connect: connection refused"
}

my question is: the possibility that this error is occurring could be some firewall blocking my connection? for example the firewall where I am hosting the database is blocking the connection or Is a firewall where I'm hosting my backend blocking the connection?

thanks in advance, if you need more info I'll be happy to provide