I use the Private Internet Access VPN provider, which allows you to request forwarding of a single port from the public IP to your machine. It's a cool feature that technically allows you to run any kind of server application through an anonymized connection, though its primary use case is obviously to make your torrent client directly connectable.
But I just couldn't get it to work. The PIA client claimed to have a port forward set up, but no traffic came through when testing it; any request I made just timed out. I exchanged about 15 emails with their support team, who gave no helpful answers to my questions and repeatedly sent me their µTorrent guide, screaming THIS FEATURE IS FOR TORRENTING ONLY AND WE DON'T KNOW ANYTHING ABOUT LISTENING SOCKETS. But after a while I found a quirk that is apparently documented nowhere:
If you run your server and client application from the same machine, port forwarding won't work.
It turns out the feature had (probably) been working all along, but I was testing it by listening and issuing requests from the same machine, through the VPN, to its own public IP. As soon as I used another machine as the client, everything worked as it should.
I don't know if this is just an issue with my network, a bug in PIA or my own stupidity acting up, but it's an entirely non-obvious detail that I currently can't explain. For good measure, I asked PIA support if this was a known limitation and they responded with this:
Port forwarding is only used in torrenting to speed up slower downloads. Any use of port forwarding outside of that is not supported nor recommended.