Unable to remove index.php on IIS 10

I have had no luck removing index.php from site's URL. I'm upgrading to 3.1.9 from 2.1.4, and any URL rewrite I try in my web.config file gives me a 500 error. My setup is:

• Windows Server 2016
  
• IIS 10
  
• PHP 7.1
  

In addition, the only way to get any controller to load, besides my default controller, is to make the following adjustment in my config.php file: 

Code:

$config['index_page'] = 'index.php?';

I got that from the Codeigniter troubleshooting page. https://www.codeigniter.com/user_guide/i...oting.html

Any help on either issue would be greatly appreciated.