As mentioned above, the Wifi does not work on any devices unless the VPN is turned on (I actually had friends over to test the internet with and without VPN). This started happening since the network fault last Friday, what is even happening.
I'm not an expert on this but what might be happening is that your Internet is connected but your modem/router is failing authentication with Optus' DNS server. Try checking the username and password settings in your modem/router in regards to your connection to Optus. If you changed any account details with Optus recently, that may be part of the problem. If fiddling with this doesn't fix things, you could try sending your DNS requests to a different DNS server, I'm not sure how you would do this but supposedly it can be done.
check your VPN software settings, there could be a kill switch option. Meaning, the system will kill the internet if VPN is not active.