Just to check whether you require NAT for the Point to Point WAN connection ?
If you require NAT for the WAN interface then you can define the separate IP subnet as “Additional Public IP Settings” for the WAN. For more information, please refer to the attached screenshot:
After defined the “Additional Public IP Settings”, you can use NAT Mapping or Inbound access for the Inbound traffics or outbound traffics NAT.
Else if you only require routing for the WAN connection, you can enable IP forwarding for the Routing Mode.