I'm using 32400 for both internal and external.
In my case, an automatic firewall rule on the destination Plex server was blocking it, not the R1. I'd be looking at the firewall settings on the Synology as the cause.
I think you don't know how to read carefully what I wrote the first time. You already replied twice without knowing what you are talking about. Please, let Netduma tech support help the customers.
None of my products have problems between them except the Netduma. I added it between my modem and my Nighthawk router (now in AP mode). Since I bought the R1, my Synology DiskStation gave me headache to address UPnP forwarding. I received everyday an email from the server about not able to port forward via UPnP. With my Nighthawk ZERO ISSUES. I closed every possibility until 2 days ago. A big learning process from the Netduma and a big waste of time to investigate what I thought was flawless.
The conclusion, I needed to disable the UPnP server and use the Port Forwarding. Everything works at 100 % for 72 hours, now. I can use the port 32400, without issues. My frustration is, if I needed to change the port of some my services, I needed to manually add a Port Forwarding rule. The beauty of my Synology NAS is effortless. Now, I need to do one. This is not dramatic because I play with computers and networks for 25 years. This is my daily job too, but I will NEVER recommend the Netduma to my friends, because your product is for advanced users only. I think, from the screenshots I saw, the DumaOS will be easier and more convenient for much more people. I will rerate your product after I will install the DumaOS on it. A summer release?
My message to Netduma, I hope you have fixed the UPnP in the upcoming DumaOS, because this is not fun to troubleshoot. For the most common humans they would have gave up, rate your product 1 star and sell it. Right now, I rate it 3 out of 5.
PS.: I never receive emails when someone replied, this is not fun. I'm subscribed / following this thread.