ZeroTier (ZT) is open source and free to use for most cases (commercial/proprietary use makes it not free).
It's a network virtualization platform that lets you connect to your devices with automatic end-to-end encryption. Think of it conceptually like a VPN, SDN, and SD-WAN all in one.
Create and verify an account over at https://my.zerotier.com/
Create a network https://my.zerotier.com/network
Download ZT on your devices. https://www.zerotier.com/download/
Add your devices to your network as members (2 or more to test it out!).
curl -s https://install.zerotier.com | sudo bash
Unix Client Commands
Get your ZeroTier address and check the service status
sudo zerotier-cli status
Join, leave, and list networks
sudo zerotier-cli join <network-id> sudo zerotier-cli leave <network-id> sudo zerotier-cli listnetworks