The Plusnet Technicolor TG582N router is odd in various ways (like the Thompson routers BE used), has an annoyingly slow and idiosyncratic Web control interface, with all sorts of facilities you might want only available via a telnet CLI interface, but:
Does its IP set up match your set up? The router's default IP address is 192.168.1.254.
What happens when you access http://192.168.1.254 - you should get a login prompt.
Maybe phone Support for advice?