I found the solution in using another cordova plugin for admob. I used it in my game (with Cordova CLI) and the ads are working fine.
I made a Construct 2 plugin for it, which works in the same way as the original AdMob plugin.
dropbox.com/sh/1hv0s03feivy1ca/AADyU_Ko8xgRtHx6ALSdYqjqa
Fist of all, you need to setup Cordova CLI on your PC. You should follow this tutorial (please follow it only until the "EXPORTING FROM CONSTRUCT" section):
construct.net/en/tutorials/building-mobile-apps-locally-with-cordova-cli-21
Then, follow these steps:
1. Export the game from Construct 2 using Cordova export
2. Go to the exported folder and open config.xml file. You should add this code (after other <plugin> tags):
<plugin name="cordova-plugin-admob-free" >
<variable name="ADMOB_APP_ID" value="Your-admob-app-id" />
</plugin>
In the value="" you should put the ID for your app from the AdMob website
3. Open Command Prompt
4. go to your exported folder (where your game is exported)
5. type: cordova prepare
6. wait for that to finish and then type: cordova build
After that is complete, you will have your .apk file. (the folder which has the .apk will be displayed in Command Prompt)
If you want to make a release apk (for the Google Play), you can follow this tutorial:
youtube.com/watch
Let me know if you need help though :)