Unfortunately unless you didn’t reboot the NAS at this time the only way to recover your file is either from an external backup or by paying the ransom.
Once you paid a quick way to recover is to run 7z from an ssh shell.

(I installed tmux in order to let the command run even if I’m not in the network)

ssh to the QNAP as admin user and run the command below, replacing YOURPASSWORD with the pass obtained from the website.

Run these commands at your own risk, never blindly trust what you find on internet! I’m not responsible for mistyped commands or if they’re not compatible with your configuration.

find /share/CACHEDEV1_DATA -type f -name "*.7z" -exec bash -c '7z x -y -pYOURPASSWORD -o\"$(dirname "{}")\"  "{}"' \;

This will find all 7z files and decompress them in the original folder.

Once done you can delete 7z files and !!!README.txt.