Unity Advertisement Banner not showing in editor

I’m trying to place an ad banner in my games main menu but it doesn’t show up in the editor.
I installed Unity ads through the package manager (version 3.4.2) and implemented everything as shown here: Banner ads for Unity developers | Advertisement | 3.4.9

I also enabled test mode in the services tab and in the online dashboard project settings. There is still no test banner ad showing when I start the game. It seems to be stuck in the while loop checking if the advertisement is ready.

using UnityEngine.Advertisements;

public class AdLoader : MonoBehaviour {

    public string gameId = "[here is my gameid from the dashboards project settings]";
    public string placementId = "bannerPlacement";
    public bool testMode = true;

    void Start() {
        Advertisement.Initialize(gameId, testMode);

    IEnumerator ShowBannerWhenReady() {
        while (!Advertisement.IsReady(placementId)) {
            yield return new WaitForSeconds(0.5f);


The console just outputs “trying…” as I already mentioned above. There are no errors or warnings.
When I build the game to android the ad banner also doesn’t show up.

Did you find any solution?