Have you ensured that your port is forwarded via your home router?
The standard RStudio port is 8787, and you'll need to make sure your home router is set up to allow incoming access to your server {internal.ip:8787}. It is usually labeled under Port Forwarding in most routers.
Also, be sure that your server allows incoming connections. This would be done via the firewall (ufw). It sounds like it's set up fine, since you can access the server internally. But it's worth making sure!
I hope this helps 