you must have internet and access_network_state permissions in androidmanifest.xml

You need to add below 2 lines into AndroidManifest.xml file

<uses-permission android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.company"
          android:versionCode="1" android:versionName="1.0">
  <application android:icon="@drawable/icon" android:label="@string/app_name"
               android:debuggable="true">
    <activity android:label="@string/app_name" android:name="BannerExample">
      <intent-filter>
        <action android:name="android.intent.action.MAIN"/>
        <category android:name="android.intent.category.LAUNCHER"/>
      </intent-filter>
    </activity>
    <activity android:name="com.google.ads.AdActivity"
              android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/>
  </application>
  <uses-permission android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
</manifest>