TPG should publish the proper settings for Sagemcom unit for local wifi operations. Eg. smart lights and switches, aircon and solar converter control panels, "casting" to TV. CG-NAT is not the issue here because that affects the external network, not local operations.
Band steering: does Sagemcom do this or not, If it does, how is it disabled? SSID names: can the 2.4GHz and 5GHz bands be given different names? How? AP isolation: how is it disabled? Every other router can disable it. Apps on phones/tablets have to be able to communicate with smart wifi devices. AP isolation prevents this. Strangely enough, ethernet devices can communicate with wifi devices. If these restrictions can't be overcome, TPG should advise users with these types of smart devices to stay away from 5G wireless broadband (but not because of CG-NAT!).
The idea of using another router as an access point to attach smart devices is only half a solution. User possibly has to buy another router, position it, power it, connect it to Sagemcom. Although, even an old ADSL router could be used for this purpose.