It's totally free, and one of the easiest way to deploy it on the server AND clients. Fully safe and secure, and NO fiddling with all those certificates and keys in OpenVPN.
Install, configure a few customers (each customer needs their own section), and you hand out the correct config file to the customer (or even just QR code if the client is on Android or iOS).
You only need to open ONE port on the firewall/router.
You can use Open VPN https://openvpn.net/
Documentation https://openvpn.net/vpn-server-resources/installing-openvpn-access-server-on-a-linux-system/
About the client: https://openvpn.net/community-resources/configuring-client-specific-rules-and-access-policies/
Quick start : https://openvpn.net/cloud-vpn/quick-start/
The free license should be enough depending of your needs.
I'm using it since 1 year and this is working fine.