Problems with IIS 7.0 installed ASP.NET 4.5 runtime

So, I have some tasks to update some programs that already run in production. The UAT and production server is using IIS 7.0 which already installed with ASP.NET 4.5 runtime, and the local development is using IIS 7.5.

Things are so smooth in the development, until I try to publish it to the UAT.

This is the error message:

0x80070032 - The configuration section 'connectionStrings' cannot be read because it is missing a section declaration.

I then change the Application Pool from "DefaultAppPool" to "ASP.NET v4.0", well that fix the problem.

But, the web app's CSS and Javascript doesn't work. Well maybe that because the development is in 4.5 that using WebOptimization features such as bundling and minification.

The solution now is to set the debug=true in web.config, and it can work properly.

I think this solution is not the best for deploying the web app. I still don't understand why it can only run in application pool ASP.NET v4.0, I always deploy my apps using "DefaultAppPool" before. And why the css and js isn't working? Can anyone explain me this about problem, or give a best practice solution?