What devices — server-grade gateways, routers, low-grade NAT-only boxes — are located between your OS X Server box server and the Internet?
...This device can control what connections reach your server.
Do you have a static IP connection and a server tier of service configuration with your ISP, or are you attempting to use a residential/dynamic IP address for your server?
...Various ISPs can and variously do block in-bound server-oriented connections and protocols — such as HTTP/80 and HTTPS/443 connections — from their lower tiers of service.
What OS X or OS X Server version are you using?
...Web services management does differ across the recent OS X Server versions.