Creating custom tab with ultimatemember plugin of wordpress

Let me share my similar experience. First of all in this code :

$tabs['mycustomtab'] = array(
  'name' => 'My custom tab',
  'icon' => 'um-faicon-comments',
 );

You should use always

mycustomtab

as key which I see you've already used. So that's true. Generally it works out when you put this code in your active theme's functions.php But if it doesn't work out, consider adding this to core file um-filters-misc.php in the plugin core file folder for ultimate-member. Let me know if it works for you.