I fixed a couple bugs this evening, including the netmask problem I was having. I just needed to set the ppp interface as the default interface ( netif_set_default()) and lwip routes all the packets through the ppp interface, even if its router/ppp server is outside the netmask network.
I enabled ip forwarding on Mac OS X to give public access to my Mac: sudo sysctl -w net.inet.ip.forwarding=1, and set up port forwarding on my router.
And so, here it is, on the internet for the first time: my
Macintosh 512K Web Server Running System 2.0 and Finder 1.1g (http://18.104.22.168:8080). It is just using the default webpage right now - I need to set up a better one. It seems to work from my iPhone. It is 30 years behind on security updates, so please be kind to the old thing!
Leave a comment if it works for you. I'll try to get a video up this weekend with more details.