Is Syncthing viable if I can't modify firewall/router settings?

I’ve used BitTorrent Sync to keep some directories synced between a couple stationary computers and several laptops - mix of Windows and Linux. Trying to switch to Syncthing but not sure it’s a viable solution.

BTSync allows me to sync over the internet with basically the default configuration - very rarely do firewall issues get in the way. But with Syncthing, everything works great over the LAN, but not over the Internet. I’ve read the documentation, and it seems that if I can’t modify UPNP, NAT and port settings on the router/firewall, Syncthing won’t work. And since the laptops are almost never on a network that I own, that’s not feasible.

Is my assessment accurate, or am I missing something?

Edit:------- Is it just necessary to have at least 1 node with a UPNP or NAT public IP, or all nodes? That would make a big difference. But I’m not really in a position to keep experimenting with this unless I know it’s feasible.

one node of two that want to connect to each other needs upnp, or the port forwarded, so one can be behind a firewall as long as the other is not.

Thanks @Alex. That makes sense but is not the impression I got from the documentation.

Use n2n v2 or hamachi, both works

The best results I get with: Select one computer (A), which is running most of the time and set it up for DynDNS + portforwarding. Use “internalname,” as adress on all other devices, e.g. laptop (B). If you are inside your LAN, the internal name/connection is used, if you are on the road the external DynDNS is used. The only disadvantage of this configuration is that sometimes if you are on an outside network Syncthing takes a little longer to connect, because only (B) knows where to connect to, but (A) has to wait for the incoming connection. Other than that it works really great and you only need 1 portforwarding. You can also connect the LAN devices at home in a row, or all to each other or just use (A) as the central “Syncthing hub”