When using the feature „Security – Security Level – Allow Connections from the list only“ you will lock yourself out, when not adding your local network.
Well, if this happened you access your device via SSH using the network which you already allowed. Then you go to
[/mnt/HDA_ROOT/.config] and edit
Add another row with your local network. This can look like this:
Reboot your QNAP and try accessing it from your local network.