How to prevent app from Crashing due to memory error?

Hello Guys,

I am facing crash issue from few days and unable to solve it. The game crashed due to "Memory Warning" after playing more than 30 minutes because of ads takes much memory. I am using Google AdMob 7.4.1 and also destroy the interstitial on closing the ad as follows:

string adUnitId = "ca-app-pub-7964334718917521/#####9";

interstitial = new InterstitialAd (adUnitId);

interstitial.OnAdClosed += HandleFullScreenOnAdClosed;

request = new AdRequest.Builder ().Build ();

interstitial.LoadAd (request);

public void HandleFullScreenOnAdClosed(object sender, System.EventArgs args)

{

interstitial.Destroy ();

interstitial = null;

Resources.UnloadUnusedAssets ();
 

}

But i think " interstitial.Destroy ();
 " is not releasing the memory just destroy the instance created. Is there any way to release the memory at runtime or any other solution. Any suggestions will be appreciated.

Thanks in Advance