Seems like some AntiVirus is blocking the installation. You can try it by allowing installer through the antivirus program.

You need to debug your app to check console error and get to know the exact issue:
https://websitetoapk.com/docs/debugging-with-chrome.html

@sefaaydin I would like to inform that this is not causing any issues. The code on our website is entirely correct and tested. However, we are still glad that you resolved it anyway.

Since this is your specific issue, please raise a support ticket:
https://websitetoapk.com/support/submit_ticket

Usually if they don't keep clicking on your ads intentionally, there won't be any issue and no need to list each of them as test device. Further, you can connect to Google AdMob support if you have some exceptional case.

First of all, you should have Material+ version, after that, follow below tutorial:
https://websitetoapk.com/docs/setting-up-firebase-fcm.html

You cannot do this to someone else's site, please do create for your own sites only.

No, you don't have to go back to homepage, you can double press quickly on any page to exit the app.

Yes, still double press the back key to exit from the app.

You need to debug your app to check console error and get to know the exact issue:
https://websitetoapk.com/docs/debugging-with-chrome.html