This link describes what you need to do fairly clearly, and gives the necessary warnings. Providing remote access to a machine on your local network is pretty dangerous.
https://www.qnap.com/en/how-to/faq/a...qnapcloud-link
Thinking about it a bit more, if you can connect to the NAS via the QNAP cloud, the basics of your network setup must be correct.
If you tell us how you’re trying to connect, that will help with which ports need to be forwarded. Eg. Are you trying to copy files, stream movies, access web pages etc.
Do you have a static IP address on your Livebox? If not that complicates things.