Styles don’t load correctly. Insecure content

__DIR__ returns the current file system path, not a URL. So the resulting stylesheet will not be a valid URL, and will not have https://, causing the insecure content warning.

To get the URL for a stylesheet in your plugin, you need to use plugins_url():

wp_register_style( 'estilo', plugins_url( 'assets/css/estilo.css', __FILE__ ) );

The use of __FILE__ here is described in the documentation for the function:

A full path to a file inside a plugin or mu-plugin. The URL will be
relative to its directory. Typically this is done by passing __FILE__
as the argument.