The issue: Toggler icon does not display in mobile view.

OMG, I spent hours looking for the answer and it was as simple as a stylesheet.

By default, Bootstrap advises to use a MIN stylesheet. The toggler csss is included with the full stylesheet:

• Not: bootstrap.min.css
• Change to: bootstrap.css

Bootstrap 4 Stylesheet HTML

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">

More information about stylesheets can be found at getbootstrap.com

More information about navbars can be found here