Your question intrigued me, so I'm disappointed you didn't get a direct answer. I downloaded the manual for your router and read it. I have a number of thoughts to share with you and I'm hoping one of them hits the nail on the head.
I manage several routers for different companies so I tried your scenario on a commercial (Watchguard) router. I was able to make it work. But when I tried the same scenario on a personal home type router(a Netgear Nighthawk) i was not successful. I witnessed the same problem as you describe. My gut feeling here is that home routers wont allow the same service, in this case RDP, to be used twice for a given WAN IP Address. In other words, you can use the service and route it to one device only, regardless of incoming port number. After I removed the working port forwarding(in your case port 8389) the non-working forwarding(in your case port 8489) began working. Try that, if you haven't already. I think it will prove my point. If it does, then the answer to your question is your router doesn't support it.
Something else; your router logs port forwarding. You should be able to find some answers in the logs about what is going on. Go to Advanced settings/system logs/port forwarding.
Another suggestion; You have a server, you don't say what it's purpose is, but if it is not setup as a domain controller then you could set it up as a VPN server/gateway. VPN in, establish a connection, and then access either device with RDP. Make sure VPN Passthrough is enabled on the router.
I hope this answers your question, or at least sends you down the right road to success. Good luck.