The good news is that with FTTC you can keep your existing network structure, you just plug your main router into the NBN box directly, you don't need to use the 5366 for internet except for when you first get the NBN connected and with the 5366.
As for phone its slightly more involved but shouldn't be too much of an issue. Just connect the 5366 to the switch, give it a fixed IP address, disable its wireless if you don't need to use it, and follow info here.
@NarimanH I think the simplest way for you to move forward is to follow the standard set ups for both devices. Firstly connect your F@ST5366TN as per the Optus instructions including telephone. Secondly turn off the Wi-Fi by pressing the Wi-Fi button on the F@ST5366TN. Thirdly RJ45 cable connect the Internet (WAN) on the Google Router to anyone of the four LAN ports on the F@ST5366TN. This set up follows the recommendations of both Optus and Google and would also assist in support from either organisation. If there is a valid technical reason for deviation then of course such customisation would have to be considered. I would be intrigued if such a requirement existed in this situation.