Two options could be helpful for you:
If you want the very same PC to be accessible from the Internet, then you will have to configure your local router to accept external connections. In this sense, your ISP should better help you with the configuration steps.
Another option would be launching an OpenProject Cloud image https://bitnami.com/stack/openproject/cloud. This installation would be different from the one you have in your local network but would be directly accessible from everywhere without having to configure anything in your local netowork.
Hope it helps.
Javier J. Salmerón