I Tried to reproduce the same in my environment to redirect the IIS app to Offline Page during application Maintenance :
I have created IIS Web Application in Windows Server with HTTP Redirection as below.

Once the IIS Setup has been completed, deploy the application code to IIS Server.
The application is running successfully.

Now add an HTTP redirect rule to IIS Server which will redirect user requests to the maintenance page as shown below.
Open HTTP Redirect option and add your maintenance page address.
Make sure to enable "Redirect all requests to exact destination".

After adding the HTTP redirect rule in IIS Web Server, whenever users hit the application. it will redirect the requests to the maintenance page automatically.

Refer to this Stack link created by : Paul Grimshaw