Port 80 appears to be being blocked by my CM500V cable modem. When I go on to the modem user interface on 192.168.100.1 I am unable to configure any port forwarding or firewall rules.
I have removed all other devices from the modem and am still unable to access my lan via port 80 from outside.
All other ports I have tried are fine incoming and outgoing. Also, I have internet access, so outgoing access via port 80 is not a problem.
The CM500V is a modem only, not a router, so there is no port blocking on the modem itself. The external (public) IP is passed to whatever device you have plugged into the WAN port - which is normally a Sagemcom router that Optus will have sent you.
So you need to do your port forwarding on either that or another device you have connected to your modem.
Be aware that Optus has previously blocked a number of ports (including 80 and 25 inbound, among others) because the T&Cs had in there somewhere that the internet service was not for "unattended services" (ie no servers). So you will probably need to setup whatever you are trying to access on a different port, and then do the forwarding on your router.