Ionic - Login authentication from Rest API

I was recently learning how to use Ionic and I was trying to make a login page for user authentication by calling an Api.

My problem is that I have not found any tutorial, nor guide, sufficiently clear and updated. Not even in the official documentation is explained in detail. So I do not really know how to do it. I leave my code to see if someone can guide me a little.

I used link as an api to test.

Rest.ts:

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';



@Injectable()
export class RestProvider {

username: string;
loginURL = 'https://jsonplaceholder.typicode.com/users';
access: boolean;


  constructor(public http: HttpClient) {
  }

public login(credentials) {
    if (credentials.username === null || credentials.id === null) {
      return Observable.throw("Please insert credentials.");
    } else {
      return Observable.create(observer => {
        this.http.post(this.loginURL, JSON.stringify(credentials))
        .subscribe(res => {
            console.log(res);
        }, (error) => {
            console.log('por aquí');
            console.log(error);
        });
      setTimeout(() => {
        observer.next(this.access);
      }, 500);
      setTimeout(() => {
        observer.complete();
      }, 1000);
    }, err => console.error(err));
  }
}

Login.ts

import { Component } from '@angular/core';
import { NavController, LoadingController, Loading, AlertController, 
NavParams } from 'ionic-angular';
import { TabsPage } from '../tabs/tabs';

import { RestProvider } from '../../providers/rest/rest';



@Component({
  selector: 'page-login',
  templateUrl: 'login.html',
})
export class LoginPage {

  loading: Loading;
  credentials = { username: '', id: '' };
  isLogged: boolean;

  constructor(public navCtrl: NavController,
              public restProvider: RestProvider,
              public loadingCtrl: LoadingController,
              private alertCtrl: AlertController) {
  }

  ionViewDidLoad() {
    console.log('ionViewDidLoad LoginPage');
  }

  doLogin() {
    this.showLoading()
    console.log(this.credentials);
    this.restProvider.login(this.credentials).subscribe(allowed => {
      if (allowed) {
        console.log(allowed);
        this.navCtrl.setRoot(TabsPage);
      } else {
        console.log(allowed);
        this.showError("These credentials do not match our records.");
      }
    },
      error => {
        this.showError(error);
      });
  }

showLoading(){
    this.loading = this.loadingCtrl.create({
        content: 'Authenticating...',
        dismissOnPageChange: true
    });

    this.loading.present();
  }

  showError(text) {
    this.loading.dismiss();

    let alert = this.alertCtrl.create({
      title: 'Fail',
      subTitle: text,
      buttons: ['OK']
    });
    alert.present();
  }

The code itself does not give an error or anything, but when authenticating what the "allowed" returns is an "undefined" and I do not know how to proceed. Thanks for the help and for your time.