App crashes after FirebaseAuth.DefaultInstance

I use FireBase authentication with a simple function that signs up a new user, it works great on the Unity Editor, but when I build it on my Android, it crashes the second it tries to read this line- FirebaseAuth auth = FirebaseAuth.DefaultInstance

This is the error I get before it crashes-

Java class com/google/firebase/auth/FirebaseAuthWebException not found. Please verify the AAR which contains the com/google/firebase/auth/FirebaseAuthWebException class is included in your app.

1. Why do I even get the Firebase-AuthWeb-Exception ?
2. Why can't it find that exception? (I tried to resolve jar files, and to re-import auth SDK but it didn't solve the issue

My app is simple- there's a SignUp button that calls this function-

 public void SignUp()
 {
    FirebaseAuth auth = FirebaseAuth.DefaultInstance;  //The error is after this line

    auth.CreateUserWithEmailAndPasswordAsync(email.text, password.text).ContinueWith(task =>
    {
        if (task.IsCanceled)
        {
            Debug.LogError("CreateUserWithEmailAndPasswordAsync was canceled.");
            return;
        }
        if (task.IsFaulted)
        {
            Debug.LogError("CreateUserWithEmailAndPasswordAsync encountered an error: " + task.Exception);
            return;
        }

        // Firebase user has been created.
        Firebase.Auth.FirebaseUser newUser = task.Result;


        Debug.LogFormat("Firebase user created successfully: {0} ({1})",
            newUser.DisplayName, newUser.UserId);
       // SignInWithVerification();
    });


}

• Unity- 2108.2.12f1
• Build system- Gradle
• Firebase unity sdk 5.4.0 dotnet3