<<< it's because the PC is in the attic and the modem and router are downstairs >>>
You can always take the router upstairs and plug it into a PC - it doesn't need to be connected to the Cable Modem to configure the wireless security.
<<< Also, there's a web-cam connected to the PC but the IP address changes every time I switch on. Any idea how I get round this? >>>
Depending on the firewall software you are using, you may be able to configure it to allow access to the complete 192.168.11.0 subnet. Changes of IP addresses wouldn't then matter.