By the end of this post, you'll be able to run:
ssh $machine_name from anywhere in the internet-connected planet, using SSH keys. It is free and requires no future maintainance. This guide uses Cloudflare Tunnel, a service by Cloudflare with a free-tier. It will filter traffic to your machines through Cloudflare's network, including authenticating you. Because of this, your machines won't directly be exposed to threat actors and "1337 haxors".