No plugin will give you an online save system unless it is integrated with a third party. For example, you can try to use firebase in your project but it not only requires Construct 3 knowledge, but also javascript, html and possible CSS for the login UI.
I developed my own plugin for firebase integration and may place it on itch.io on sale... still in dev, so whenever it is ready I'll post it there. You can try to develop your own, once you get it is not too complicated but as I stated, you have to learn more than only Construct 3.
Take a look to firebase documentation:
firebase.google.com/docs/web/setup