After moving the server software to a different machine (macOS -> macOS) the new server, after enabling remote dvr/automatic -- selected port 8090 for external access, likely because 8089 was in use by the old server. After decommissioning the old server, rebooting the router and the new server, the new server still insists on using port 8090 "forever" -- even though 8089 is available. This situation causes issues for appleTVs connecting remotely as they want to use port 8089... How do I fix this? A workaround is to manually forward tcp port 8089 on the router to the local server IP address on the lan. Even with this manual workaround in place, [NAT] activity in the logs is still shows "Successfully mapped port 8090 using natpmp"
If you're adding an 8089 port forward rule to the router yourself, then the DVR software should be set to Manual mode in the Remote Access section.
Yes that is what I did and it works. Now how do I get the server software to use port 8089 "automatically" again. It is stuck using 8090 and will not revert.
This is up to your router, not the DVR. The DVR asks for 8089, and uses whatever your router responds with. In this case it's responding with 8090. If you have added 8089 yourself to your router, then it sees that's not available and adds one to get 8090.
You don't really need automatic mode. You can leave it on manual and setup your router yourself with a static DHCP reservation and the 8089 port forward.