Template for custom content type
To recognize a suggestion in a module, you need to explicitly specify it in hook_theme() of your module.

system_theme() for example has this:

// Normally theme suggestion templates are only picked up when they are in
// themes. We explicitly define theme suggestions here so that the block
// templates in core/modules/system/templates are picked up.
'block__system_branding_block' =&gt; array(
  'render element' =&gt; 'elements',
  'base hook' =&gt; 'block',
),


Copy the original definition and then change the key and set the base hook.