How to show interstitial ads every 40 seconds in flutter dart

Question

I have a working code of interstitial ad with Admob in flutter, however, the ad can only be seen when the user clicks on the button. I use a timer to show the ad every 40 seconds, it prints in the console that Interstitial ad is loaded but the ad doesn't show on the device. Just want the ad to show every 40 seconds, thanks

import 'package:google_mobile_ads/google_mobile_ads.dart';
import 'dart:io' show Platform;
import 'colors.dart' as color;
import 'dart:async';

class adsinter extends StatefulWidget {
  const adsinter({Key? key}) : super(key: key);

  @override
  _adsinterState createState() => _adsinterState();
}

Timer? _timerForInter;

class _adsinterState extends State<adsinter> {
  //add Banner ad
  InterstitialAd? interstitialAd;
  bool isLoaded = false;

//   @override
//   void initState() {
//     // TODO: implement initState
// // Add these lines to launch timer on start of the app
//     _timerForInter = Timer.periodic(Duration(seconds: 20), (result) {
//       interstitialAd;
//     });
//     super.initState();
//   }

  @override
  void dispose() {
    // Add these to dispose to cancel timer when user leaves the app
    _timerForInter?.cancel();
    interstitialAd?.dispose();
    super.dispose();
  }

  @override
  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();

    _timerForInter = Timer.periodic(Duration(seconds: 10), (result) {
      String? getBannerAdUnitId() {
        String adiOS;
        String adAndroid;
        if (Platform.isAndroid) {
          // Android-specific code
          return "ca-app-pub-2014810929195140/8666570300";
        } else if (Platform.isIOS) {
          return "ca-app-pub-2014810929195140/7584778723";
          // iOS-specific code
        }
        return null;
      }

      InterstitialAd.load(
          adUnitId: getBannerAdUnitId().toString(),
          //"ca-app-pub-3940256099942544/1033173712",
          request: AdRequest(),
          adLoadCallback: InterstitialAdLoadCallback(
            onAdLoaded: (ad) {
              setState(() {
                isLoaded = true;
                this.interstitialAd = ad;
              });
              print("Interstitial Ad loaded");
            },
            onAdFailedToLoad: (error) {
              print('InterstitialAd failed to load: $error');
            },
          ));
    });
  }

  @override
  Widget build(BuildContext context) {
    return IconButton(
      icon: Icon(
        Icons.loop,
        size: 30,
        color: color.AppColor.loopColor,
      ),
      onPressed: () {
        interstitialAd!.show();
      },
    );
  }
}