If a specific app doesn't require full network access permission means it's completely privacy safe? If they can't access internet, they can't steal my data in the back which is safe right? I wanna try this file encryption app from play store. Before i proceed, i wanna make sure that the app itself is completely safe. It's called SSE encryption app.
Unfortunately, all apps always have the ability to access the internet. The permission can be misleading. It has to do with viewing other devices on a network and the network details.
What you can do is right after downloading the app, put your phone in airplane mode. That will prevent any data from leaving the phone, then uninstall it before turning airplane mode off.

Although in all reality, unless you're running Paranoid Android, your data is never truly safe, and even then it isn't perfect.