Unity ads invalid configuration request for gameid

Hi all,

I'm currently trying to integrate and test a simple banner ad on my app, and seem to be stuck with these errors appearing when initialising the ads:

• Invalid configuration request for gameId: xxxxxxx

• The remote server returned an error: (404) Not Found.

  ---

I have been following the basic tutorials for this setup, and have my placement and projects set up on the unity dashboard, as well as the following code within the unity environment itself:

 private string _androidGameID = "xxxxxxx";
         private string _iOSGameID = "xxxxxxx";
 
         private string _gameID { get
             {
 #if UNITY_IOS
                 return _iOSGameID;
 #else
                 return _androidGameID;
 #endif
             } }
 
         private string _bannerID = "bannerPlacement1";
         public bool TestMode = true;
 
         public void InitializeAds()
         {
             Advertisement.Initialize(_gameID, TestMode);
             StartCoroutine(ShowBannerWhenReady());
         }
 
         IEnumerator ShowBannerWhenReady()
         {
             while (!Advertisement.IsReady(_bannerID))
             {
                 yield return new WaitForSeconds(0.5f);
             }
             //Advertisement.Banner.SetPosition(BannerPosition.TOP_CENTER);
             Advertisement.Banner.Show(_bannerID);
         }
     }

This being initialised in a Start method in my main game class as so:

 private UnityAdHelpers _adHelper;
 
     void Start()
     {
         GameConstants.SetGame(this);
         _adHelper = GetComponent<UnityAdHelpers>();
         _adHelper.InitializeAds();
     }

I have the testing modes enabled on both the project settings int he dashboard and the ads area in the Editor. After trying to find results for a long while online, I am at my whits end here, any advice?