{#if $request.isMobile}}   
      <h1>This is mobile specific title</h2>
{{/if}}

{{#unless $request.isMobile}}    
      <h1>This is desktop specific heading</h2>
{{/unless}}

If you want to use if/else condition for mobile detect, you should use it like follow.

<div class="container">
{{#ifeq $request.isMobile 'true'}}     
      How to add mobile specific code ?
      <div>
      <h1>This is mobile specific title</h1>
      </div>
{{else}}
      How to add desktop specific code ?
      <div>
      <h1>This is desktop specific title</h1>
      </div>
{{/ifeq}}
</div>