Admob banner ad (even the test ones) not visible in Ionic 4 Android app

I am trying to integrate Admob ads with my Ionic 4 (Angular) Android app. So, I installed the plugin by executing the following commands.

cordova plugin add cordova-plugin-admob-free --save --variable ADMOB_APP_ID=<..MY APP ID HERE...>
npm install @ionic-native/admob-free --save

After that, I put the AdmobFree service in the app.module and put the below things in the app.component.ts

 initializeApp() {
    this.platform.ready().then(() => {
      this.statusBar.styleLightContent();
      this.statusBar.backgroundColorByHexString('#4d4d4d');
      this.splashScreen.hide();
    });

    const bannerConfig: AdMobFreeBannerConfig = {
      isTesting: true,
      autoShow: true
    };
    this.admobFree.banner.config(bannerConfig);

    this.admobFree.banner.prepare()
      .then(() => {
      })
      .catch(e => console.log(e));
  }

Then I built the app following Ionic’s documentation. However, the test banner app is not appearing at all in the mobile device. What is possibly going wrong?