I have just signed up to Optus 5G Home Broadband here in Australia. The modem/router provided by the ISP (Optus) is the Nokia Fastmile 5G.
It has 1 WAN port and 3 LAN ports. It also broadcasts a wifi signal for devices to connect to. The Nokia Fastmile 5G router address is 192.168.1.254 and all internal IP's are automatically assigned between 192.168.1.100 - 192.168.1.200. It is running a DHCP server which cannot be turned off. These settings cannot be changed - they have all been locked by the ISP.
Furthermore, there are no setting for OpenVPN client or OpenVPN server, so I'd like to use my pfsense box for that.

Is this possible? On the Nokia router, should I plug the pfsense box into the WAN socket or one of the LAN sockets?

On the pfsense box, what should the interface setting be? Static IPv4? DHCP? Something else?

Any advise/ideas would be appreciated.

Thanks