Gradle sync fails to get AWS Mobile Client library in Android Studio

I have a project loaded in Android Studio 3.0. Gradle sync works fine, and the project builds.

When I add implementation 'com.amazonaws.aws-android-sdk-mobile-client:2.6.+' to my build.gradle (Module:app) file, right next to all the other dependencies that are already part of this fine project, gradle fails to find that dependency. Many of the existing project dependencies are under com.amazonaws.aws-android-sdk-* and are being sync'ed just fine, for e.g. implementation 'com.amazonaws:aws-android-sdk-core:2.6.+' is fine.

So I double check that new project dependency actually exists, browsing http://repo.maven.apache.org/maven2/com/amazonaws/aws-android-sdk-mobile-client shows it exists, I don't see a typo.

Looking at my build.gradle (Project: myProject), I see the following

allprojects {
  repositories {
      mavenCentral()
      google()
      jcenter()
  }
}

Yet when gradle syncs i get

Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.amazonaws.aws-android-sdk-mobile-client:2.6.+:.

Could not resolve com.amazonaws.aws-android-sdk-mobile-client:2.6.+:.
Required by:
project :app
No cached version of com.amazonaws.aws-android-sdk-mobile-client:2.6.+: available for offline mode.`

uncheck "Offline Work" in Android Studio / File / Settings / Build, Execution, Deployment / Gradle