The modem itself is a simple hard-wired device with no switches or user adjustments available, and it's connected to the pc via USB. There is no LAN involved.
It would have been helpful to have
started with this information!
OK, so you can ping ... what?
Can you ping
www.pprune.org? Does it resolve the name to an IP address?
If it does, then DNS is not the problem - it's a browser problem. If not, then it's a DNS problem - assuming that you get a response back when you ping 213.161.73.143.
What happens if you paste
http://213.161.73.143 into your browser?
In IE, go to Tools, Internet Options, Connections - what's listed? What are the LAN settings - is there a Proxy Server set up? If so, disable it. Ditto for Dial-up and VPN settings.