Below are the most common reasons that users seem to have trouble with an ssh connection.
- You haven't enabled forwarding or have enabled it incorrectly
If you don't enable port forwarding from the router, or have configured it incorrectly, the connection will be refused. Instructions on this will vary based on the router, but ensure that the forwarded IP address is the same that the RPi is using.
Also, note that the address can change from time to time. I have my router set to always give my RPi the same address because of this.
- Your ISP may disable traffic on port 22
I have never experienced this personally, but some networks block "common home server ports", such as 21 and 22. If they do, there's nothing you can do to enable those ports. Switching to a different port will usually solve this problem, even if it is a bit annoying.
- You're trying to connect from the same network
Some routers don't like it when the destination is within the same network. The Motorola I previously rented from Comcast would fail with the same error, but my TP-link manages the connection without an issue.