Communicate backend with frontend in railway

Question

I have an app in railway that has 2 parts, the front and the backend

The backend tells me to use that to communicate internally

This is my code in frontend:

const handleSubmit = async (event) => {
    event.preventDefault()

    try {
        const result = await axios.post(
            'http://localhost:3001/auth/login',
            {
                username,
                password,
            }
        )
        console.log(result.status)

        setCookie('access_token', result.data.token)
        window.localStorage.setItem('token', result.data.token)
        window.localStorage.setItem('userID', result.data.userID)
        console.log(result.data.userID)
        if (result.data.userID === undefined) {
            logout()
            alert('Acceso denegado')
        } else {
            navigate('/home')
        }
    } catch (error) {
        console.error(error)
        alert('Acceso denegado')
    }
}

And this my backend :

router.post('/login', async(req,res) =>{
 const { username,password } = req.body;
 const user = await UserModel.findOne({ username });
 console.log(req)
if (!user){
    return res.json({ message: "user doesn't exist"});
}

const isPasswordValid = await bcrypt.compare(password,user.password);
 if (!isPasswordValid){
    return res.json({message: "user or password is incorrect"});
 }

 const token = jwt.sign({ id : user._id }, "secret");

 res.json({ token, userID: user._id});

});

An this the userschema in the mongodb

import mongoose from "mongoose";

const UserSchema = new mongoose.Schema({
 username: { type:String, required:true, unique:true},
 password: { type:String, required:true}
});
export const UserModel = mongoose.model('users', UserSchema);

What do I have to use to communicate with the backend from the front? I try with:

'http://backend.railway.internal:3001/auth/login',
'https://backend.railway.internal:3001/auth/login',
'http://backend.railway.internal1/auth/login',

And nothing, what am I doing wrong?