HOw to show icon on buttons

Question

I am applying bootstrap icon on button,

 

Problem: its now showing  ( attached is snap short ), i used below reference to apply 

https://codepen.io/jnbruno/pen/vNpPpW  ( use this link code to apply )

 

Note: on server ( windows server browser ) its showing fine , after publish when i open site on browser its not showing ( icon on buttons )

 

Solution Required : How or what to do to visible icon on browser ( i have checked on both browser i.e and chrome , icon not coming up )

 

Thanks 

Images are attached for reference