I've always found this path to be more compelling:

https://github.com/samyk/pwnat

It has more edges and doesn't handle all cases, but it also avoids the need for any kind of intermediary.

Older solution that seems to have issues with some modern routers:

https://github.com/samyk/pwnat/issues/18

https://github.com/samyk/slipstream is the new one

I didn't know about this tool, that's pretty useful!

Too bad the broken nature of NAT means this approach will just ignore any firewall rules you have configured and any malicious device or program can leverage it to open inbound connections.

But most of all, Samy is my hero.