@optimistnz in my set up the wi-fi's are disabled. But if what you have works and the wi-fi is enabled but there are no connections then it's a waste of time to find out why.
This set up had the Wi-Fi disabled on the F@ST3864AC and the only configuration after a "factory reset" would be to reserve the IP Address 192.168.0.2 for the Archer and the Web Server ports to that same IP Address. The Web Server could be viewed and maintained on the internal net work as the F@ST3864AC could accommodate loopback (hair pinning) so the set up kept Optus support on board.
However when Optus support sent me a F@ST5366TN it was confirmed by Sagemcom that default loopback was not a feature on the device. Such is life but it's not a "show stopper" so the F@ST6366TN was relegated as follows:-
As you can see there have been other devices added over time as the Home Network expands. There are two things that must be kept in mind under this approach. Firstly there is limited (effectively none) Optus support so I have the cables set so I can switch back if the need arises. The only downside is lack the up loopback but on the upside Optus can perform all their tests without challenging the user set up. Secondly it should be noted that the F@ST5366TN must first be connected to the NBN box in order to register (phone) with Optus. After this the Wi-Fi can be disabled before it is powered off and reconnected Archer LAN to F@ST5366TN WAN.
Also in the above diagram I have only shown the Reserved IP Address from the Archer for the F@ST5366TN but in reality it is still a router capable of devices connected to its network of 192.168.0.0/24.
@optimistnz I should have said that originally I set up the F@ST5366TN exactly as the previous Optus router and only converted to the final configuration when it was discovered that loopback wasn't available. In fact that is the exact set up I use when raising an issue with Optus and they check the line. So yes the Port Forwarding was set and the Wi-Fi was off with ALL other settings at "factory default" and there was no issue with power off and on. So it is a mystery but not worth the effort to dig any deeper if you have a workaround.
As for your second point the device does not need to be re-registered unless you do a "factory reset".
@optimistnz when the Archer is connected to the internet then simply connecting a LAN port to the WAN port of the F@ST5366TN allows the registered phone to make outgoing calls. With outgoing calls established the the set up for incoming calls is very simple. Firstly to ensure that the same IP Address is always assigned under the Network/DHCP Server an IP Address is Reserved for the WAN Mac Address of the F@ST5366 and tested to ensure that it has been applied. Secondly under NAT Forwarding/Virtual Server port 5060 is set up to forward to that Reserved IP Address and selecting ALL as the protocol ensures both TCP and UDP (most likely obsolete) are caught. The final entry is to disable the SIP ALG which is not on all routers. Here are the screens for the above with the Mac Addresses hidden as you need to find the one that applies to your F@ST5366 :-