you can run...
"netstat | grep -i ftpsync"
...on the server and then allow the ports that it uses to the firewall. if that doesnt turn up enough info you can google about netstat to better tune you search for active ports.
but if you are using the same protocol offsite as onsite that inst your issue unless the offsite firewall restricts requests initiated by clients inside the network which isnt a super normal thing for most routers unless otherwise configured to do so (most off the shelf routers/"modems" will automatically open ports that clients request traffic on).
for limiting bandwidth to external clients i cant help you. there might be mention in the ftpsync documentation, if you can find any. a quick google turned up a 404 and something on the Debian website about making a mirror with it.
to test that it isnt just the network being sad you can try copying a file between the networks with a different protocol, like scp, and check the speed on that. then possibly check speed domain vs. ip (if you have a domain setup).