There are tons of tools and options, including iCloud.
The challenge is that when workstations are behind firewalls, and properly protected (nearly every modern network)...they are hard to see. Most tools that are cloud based have an agent of some sort that makes an outbound connection, and removes the issue of IP addresses or DNS or any other machine discovery processes.
That's why nearly all cloud based solutions involve a server and an agent or client...and because if any machine has an intermittent connection, ideally the server as the man-in-the-middle is on, on line, and has the latest version of every file. Better up time, less issues.
There are 4 possible options, here in the general order of easiest to hardest:
1. Use a cloud based sync, like Drop Box, or iCloud, etc.
2. Use an agent that make machine connections, but with out a paid server. Something like
Sync Mate (may be as easy as option #1, but have not used it myself)
3. Get a NAS and run your
own server. Cost up front, but you own it, with no ongoing license or subscription. Synology has a nice sync tool called
Drive that is pretty slick, once you have the NAS setup. They also have a QuickConnect service which makes it easy to sync files with any internet connected devices. Bonus: you can do lots of other stuff with a modern NAS too.
4. You can configure everything manually and run DDNS, and probably port forwarding on the routers at both locations...or you could have a dedicated VPN between both routers. Either way, lots to configure and then maintain, but the cheapest option (if you don't count time and frustration).