I have the same problem. I have a Paradox MG5050, IP150 etc. I lost all access going to Optus and tech support was rubbish. I have installed "Alarmin" on my phone and that works for external access. The Paradox software died.
the earlier paradox alarm gear uses an app called iParadox and requires a port forward rule from the wan to the lan, by default it is port 10000. you dont need to forward port 80. this generally works straight out of the box, though in some firmware versions the ip150 might get confused and needs a hard reset to play nicely again. you will need either a static wan ip address or a dynamic dns service to make this work.
if you are using a 3G / 4G wan gateway for your internet connection you may not have a public facing ip address, and your provider may not wish to provision one in their NAT to you, in which case youre likely up the creek .... unless maybe provision your own vpn ??
the newer paradox gear uses an app called Insite Gold and does not require any port forwarding as it uses a virtual connection via the paradox Swan Server (bit like using teamviewer to remotely access a pc) its fairly straight forward and simple to setup. this even works when you are double NATted over a 3G / 4G.
in most cases firmware updates on your paradox hardware will achieve the newer scenario.
Cheers for the info. I had all of that set up but when we got our Optus NBN modem it failed. The modem is a pile of rubbish for WiFi, for configuring, blocked ports plus tech support.
I just about gave up until I found Alarmin and problem solved.