Correct, because as explained in the guide you were directed to your ISP is allocating you private (non-routable) addresses.
You’ve got three choices:
- Get your ISP to allocate a routable (public) IP - if this is a mobile broadband connection that may not be possible. They will likely charge for this.
- Use the NabuCasa service, which costs
- Build your own relay, which you may be able to do for free. You’ll do the magic of DuckDNS and LetsEncrypt there.