FYI, I’m also running a Netgear R7800, with a Synology DS916+. I also see that when I enable remote access from the DVR site, it does not work for some reason. I can see the port listed in the R7800 uPNP settings as expected - just no traffic is getting through.
Just to help isolate the issue, I tried setting up remote access to the Synology management console on port 5000 using uPNP, and that worked as expected (showed up in router uPNP list, and I could indeed access it).
The only other clue I can offer is when I disabled remote access from the DVR site, it did not automatically remove the port from the router’s uPNP list. I expected that it would remove it (the Synology remote access stuff does this as expected).
I should add that I’m not at all worried about this, as I use a VPN for remote access - but since I happened to have the same router, I thought I’d do a little digging to try to be helpful!