NoSuchMethodError: no non-static method

Hi, I am trying to use AdMob plugin from and the demo project works fine but when I try to implement on my own BluePrint project everything compile fine but I get this error when trying to show a banner ad:

System.err: java.lang.NoSuchMethodError: no non-static method “Lcom/epicgames/ue4/GameActivity;.AndroidThunkJava_AdMob_ShowBanner(Z)V”
05-30 16:44:31.040 23728 23749 D UE4 : [2018.05.30-20.44.31:040][193]Assertion failed: Method != 0 [File:D:\Build++UE4+Release-4.19+Compile\Sync\Engine\Source\Runtime\Launch\Private\Android\AndroidJNI.cpp] [Line: 189]
05-30 16:44:31.040 23728 23749 D UE4 : Failed to find Method
05-30 16:44:31.040 23728 23749 D UE4 : LogAndroid: Error: === Critical error: ===
05-30 16:44:31.040 23728 23749 D UE4 :

If I try other kind of ads the name of the method is different but the error is the same. I think this is a problem about not getting the proper Activity but not idea how to fix it: Moreover, I don´t understand why the very same Plugin code works on demo project but not on mine.

Any ideas please? I have lost 2 days already trying to fix this and the only thing I need are the rewarded ads, as I am able to get banners and interstitial using the regular methods. Therefore I don´t want to buy the universal mobile ads as I find it very expensive for a free engine.