The following are steps to setting up PiVPN with stunnel. Why would you want to do this? OpenVPN is subject to blocking by several methods of deep packet inspection since OpenVPN traffic, though encrypted, looks slightly different than normal web traffic. If your OpenVPN works, you probably don't need this. But if it is being blocked, you will probably have to wrap your OpenVPN connection in an SSL tunnel to make it look like normal web traffic. For an explation see here: https://proprivacy.com/guides/how-to-hide-openvpn-traffic-an-introduction
The following are steps needed to wrap your OpenVPN in an SSL connection wtih a Linux server (I got it working in ubuntu 18.04; looks to only work with amd64 architecture, not armhf). So far I've only used with a Windows client, so I don't know the exact config for an Android, iOS, Mac, or Linux client.