@TheRettom has already given you a great answer.
Since you didn’t provide much detail about how your home network connects to the internet - specifically whether your Ubuntu server is also your router or if you have a separate SOHO router - I’ll assume the latter.
The feature you’re looking for is called NAT Loopback, also known as Hairpin NAT or NAT Reflection. This allows devices inside your local network to access a service using the public IP (or a domain name resolving to it). Without NAT loopback, most routers won’t allow traffic to go out to the public IP and come back in, causing the request to fail.
Check if your router supports this feature. In the meantime, I found this Reddit post that provides a nice summary of the issue you’re facing.