Oh, from Syncthing:


Simple. Syncthing doesn’t need IP addresses or advanced configuration: it just works, over LAN and over the Internet. Every machine is identified by an ID. Give your ID to your friends, share a folder and watch: UPnP will do if you don’t want to port forward or you don’t know how.
Never open a port or use UPnP unless you have a firewall appliance in place like pfSense, OPNsense, Untangle, etc. In lieu of that I personally use ZeroTier One... ZeroTier is like a reverse VPN. You can use it to access your computer without port forwarding and opening yourself to the world of Shodan, Censys Masscan, Nmap, etc. I use it now when my Bro and I use my Teamspeak server and game over LAN games in Steam...