ng-bootstrap modal is not displaying

I faced the same problem, the issue was with CSS. When I check the elements of the document, find that the modal elements was created but not showing in the screen. Below css has solved the issue.

   .modal-backdrop.fade
   {
     opacity: 0.5;
   }
   .modal-open .modal
   {
     opacity: 1;
   }

ng-bootstrap only works with Bootstrap 4, not compatible with Bootstrap 3.