Ionic PRO login not working

Hello, I am making an app using Wordpress and Ionic.

I setup a login which works fine on browser but when I try to test the app in Ionic PRO, my login doesn’t work. It doesn’t alert any error, only shows loading screen and nothing happens after that.

I also tried viewing the app in xcode and on login I receive a 501 not implemented server error.

I tried contacting server support to see if the problem is CORS related but they haven’t been able to resolve my issue.

Here is the coded from the provider:

  postLogin(username, password){
    let data = {
      username: username,
      password: password
    };

    let headers = new HttpHeaders();
    headers.set('Content-Type', 'application/json');
    return this.http.post(this.api_url, data, {headers: headers});


  }

And in the Login page I call the function like this:

  onLogin() {
      this.presentLoading();
      this.authProvider.postLogin(this.username, this.password).subscribe(data => {
      this.navCtrl.setRoot('MenuPage');
      localStorage.setItem('wpToken', JSON.stringify(data));
      this.loader.dismiss();
    }, error =>{ alert(JSON.stringify(error))});

  }

I followed this tutorial for my login - https://www.youtube.com/watch?v=kiXs7hrmmmg&t=

I tried several different approaches but none of them made any difference.

Hope someone here as any idea what my problem is related to.