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.
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):
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" />
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:
Let me know if you need help though :)