Hi, apologies if this has been asked and answered before, but for the life of me, I can’t work this out. I am extremely new to Linux, and decided that it was time for me to make a home seedbox / plex server / cloud server / whatever else this can do, and a RockPro64 with DietPi seemed like the perfect place to start. I have had only minor issues until now, with a quick search online answered most if not all, but when it comes to using my VPN provider, I am getting nowhere. I use ExpressVPN, which suits my wife and kids as it’s installed on the Android TVs for viewing overseas Netflix, so I really don’t want to change. But I have tried and tried to get ExpressVPN to work on DietPi, but am having no luck. The installers don’t seem to work, either, I can’t extract them or they just error, and PiVPN and OpenVPN seem more geared towards providing your own VPN service rather then connecting to one. I have a .ovpn file, the credentials, but I lack the know-how to get it working. My ISP throttles any torrenting (private or otherwise) to less than 10% of my connection, so any help would be greatly appreciated. What I was hoping for was an always on VPN, that connects on startup and if possible (what I have setup in Windows) is that if the VPN disconnects, it would stop all internet traffic, but could still be accessed on the local network, and would attempt to reconnect. 
Krimlocke
You are right that dietpi-software OpenVPN sets it up as server, but it can act as client as well.
They offer an app as well, but most likely not for ARMv6 / ARM64, right? Can’t see it since one needs to sign up to access the download page.
The OpenVPN install instructions are indeed completely missing in their docs, however it should be quite easy:
apt install openvpn
mv /path/to/your/.ovpn /etc/openvpn/expressvpn.ovpn
cat << _EOF_ > /lib/systemd/system/expressvpn.service
[Unit]
Description=ExpressVPN
After=network.target dietpi-boot.service
[Service]
ExecStart=$(command -v openvpn) /etc/openvpn/expressvpn.ovpn
[Install]
WantedBy=multi-user.target
_EOF_
systemctl daemon-reload
systemctl enable --now expressvpn
This should install and start the VPN connection. You can then check status, start, stop, restart via systemctl:
systemctl status expressvpn
systemctl start expressvpn
systemctl restart expressvpn
systemctl stop expressvpn
It will start at boot automatically, to disable this:
systemctl disable expressvpn
You can also start it outside of systemd/systemctl via openvpn command directly, e.g. for testing reasons:
openvpn /path/to/your/.ovpn
Try consider NordVPN or Surfshark VPN as they are the best VPNs for Netflix (listed here) so far. Use them on trial basis and if they fulfill your need, they go for them on full buying.