As they have FTTC there is no special config required, just plug the DrayTak into the NBN box and off you go.....for internet at least.
If they require an Optus landline though then they will still need the Optus router, but it can be plugged into DrayTek and may/may-not need VoIP related settings.
What is their current Optus supplied router? If its this one, and its replaced by the DrayTek then they will miss out on 4G backup except for anything connected to the Optus router.
As for speed, are the speed tests done via wifi or LAN? Any Optus supplied router should be able to handle a 50/20 connection without breaking a sweat over LAN. I doubt that the router is faulty (though wifi isn't the best with earlier models), more likely there is either a problem with the line or with in house phone wiring.
Just to add to @YetAnotherAcc post, if your customer wants to use an Optus Landline, the landline will have to be connected to the Optus Modem. I'm afraid we don't provide the VoIP configuration settings to our customers.
If they just want to use their Optus Internet connection without a landline, as @YetAnotherAcc mentioned, you should just be able to connect the Draytek directly to the NBN box. However, Optus will only be able to provide limited support with third-party devices.
We can run some test on their NBN connection to see if there are any issues on their line, they'll just need to message us in the My Optus app for further assistance.