As the previous post confirms, if you disable the DHCP server on the BT Hub, only make connections using the ethernet LAN ports (so have nothing connected to the WAN port) and set its IP address to a suitable compatible value for your network, you are effectively turning it into a network switch and wifi mesh controller/access point like any other consumer-grade mesh wifi system running in “Access Point” mode rather than “Router” mode. The BT Hub may complain about the lack of internet service on its WAN connection when you log into its browser control panel or use the App (I have not used BT routers this way so cannot be sure), but this should not cause any issues in use.
For completeness you would then turn off wifi in the ISP-supplied router that is now the main router for your setup.