We have put together a short survey to capture your feedback in terms of what BT can do when you experience home Wi-Fi issues. We would appreciate it if you could take a couple of minutes to complete a short survey. Read more from here, Fix my home Wi-Fi
What are you actually wanting to do, you are mixing several terms. Do you want to change subnets, do you want static LAN IP addresses or do you want to change the subnet mask in order to have more than one subnet.