The Sagemcom 3834ac works well to access the internet on a tethered connection but not as a modem or anything else.
The range on the wifi is far worse than my previous Netgear Modem/Router which I thought was rubbish. 5Ghz only reaches 13m and the 2.4Ghz gets 14m on a good day, some days these will only reach 6m. This means the range extender I had in place for the previous Modem doesn’t pickup the signal and drops out continually. This extender use to work with the previous Netgear Modem/Router with no such issue.
You probably want to look at the TR-069 specification - its a standard that allows remote management of devices.
Numerous ISPs have this enabled on their supplied devices, so its not an Optus only thing.
As for Authorising them to have access - probably find its well covered in documentation in regards to having a service delivered.