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Helo,
I have an Asustor NAS. I set up a ExpressVPN (from config file) and try to hide qBittorrent behind it. I googled , found out where to set it, but
I see only these options - eth0 and lo. I see no tun0. Am I missing something ? Thanks.

The separate interface only shows up if you connect to your VPN via a specific protocol.
For example, OpenVPN connection will result in a tun0 interface.

Check your VPN providers docs if you can use OpenVPN.

If the config file was for OpenVPN, make sure you started the daemon.