I use Ninja Forms for managing contact forms in my site (www.privacidadycumplimiento.es).
When logged into wordpress, site shows correctly the form, but without login, form is not showing.
Sorry, but we did not test this plugin with the theme. Our policy states that we do not support third-party plugins or custom code, so, unfortunately, we cannot help you.
Also, we cannot recommend any plugins other than the ones we support. You can find the list on this page, in the sidebar: