Optus blocks certain port on the network level not on the router level, and they don't disclose which ports they are.
What are the port forwarding setting that you have set on the 5366?
What service/program are you trying to open up on the Pi?
What type of NBN (FTTN, FTTC, etc) do you have?
28967 doesn't appear to be a common port, so I doubt its being blocked by Optus.
You said that you get no joy pinging it from an outside source, but is it supposed to respond to ping? The target software on the Pi has to be programmed to respond to a ping. Just because a port is opened does not mean that it will automatically respond to a ping.
Have you ever used this Pi with another ISPs internet connection and have it accessible remotely? In other words, do you know that the Pi is configured correctly to be accessed from the internet?
What outside source are you using? What ISP, and what type, (i.e. fixed line or mobile)?
As you have FTTC you could temporarily connect the Pi directly to the NBN box for testing to rule out/in the 5366 as the cause of the blockage. But be warned that without the 5366 in front of it there will be no firewall protection for the Pi (and anything connected to it) so all security will be the responsibility of the Pi.