Although running a public web server isn't allowed by the TOS, Comcast doesn't generally block port 80 -- they're tolerant of hobby/personal servers.
When you were doing your testing, did you do it from outside your home network? Most routers don't do port-forwarding when the connection comes from the LAN.
Since it's just for personal use, maybe you could run it on a different port. To access it, put the port number in the URL, e.g. 18.104.22.168:8080 to connect to a server on port 8080.
It's known that the Comcast-specific firmware in their gateway devices has some limitations, maybe port 80 forwarding is one of them. We often suggest getting your own router, and having Comcast put the gateway in bridge mode.
Thanks Barmar, your suggestions were very helpful.
Part of the problem was, as you said, I was trying to access my server from within the home network. I didn't realize that the router would fail to port forward in that case, but in retrospect it makes sense.
I had also tested this from outside my home network, and it was failing apparently because my server is temporarily on wifi only (like I said, it's just for some minor hobby stuff, so wifi is good enough). The problem was that the wifi connection would terminate when the server went to sleep...I feel silly. As long as the server is awake it works
So for the record, this router does seem to successfully port-forward, at least on 22 and 80.