OS X Server no longer externally available for CalDav after Update from 10.6.8 to 10.11.6

Weird issue here, after updating my client's OS X Server from 10.6.8 to 10.11.6, I can no longer reach my calendar server externally. My client's router is port forwarding to my server 8443 and 8008 over TCP/UDP, and I've tested those ports and found them to be open. External DNS is resolving to the correct IP, and Port Forwarding is pointing to the server It all works perfectly internally. I've heard it could be an Apache problem, but I'm not sure where to start there. I haven't found anything that pops out to me in the logs, but that doesn't mean there isn't something there. Note: The whole reason for doing this was to make a smoother transition to a newer server that they purchased, but I need to get them up and running first.