|Does it have multiple lan ports or just one?|
Live in the UK?
Modems don't have firewalls. Modem/Routers do.
When you connect a pc to it can reboot then do a ipconfig [assuming you are set for dhcp client] what is the gateway ip?
The gateway ip is what you use to access its web interface.[if it has one]
search this web page for a54br using Edit and find on this page .
is this your unit?