If you ping to your Domain Name and the ping is OK (with the desired IP) we can discard that the domain name is not configured correctly.
About the port forwarding configuration: if the ping answers OK, it doesn't mean the port forwarding configuration is OK. In this case the configuration should be:
80/tcp from internet -> 8080/tcp to your MacOS Server
About if your Redmine installation was available on your network, I would like to confirm that if you go to your Network IP (and not the 127.0.0.1 or localhost URLs), you can see the Redmine installation.
I wanted you to test (and it is important, as maybe you have firewall problems on your local machine), if from the same network (so, from another computer) you can access to your Redmine installation. For example, if your MacOS Server have the IP 192.168.0.3, if the firewall configuration is configured properly, you should be able to access to your redmine installation from another PC in the same network, going to 192.168.0.3:8080
I'll be waiting for your feedback