|I'm not really sure what 3G mode is. Does the router have cellular capabilities?|
I would open command prompt, type in ipconfig, then hit enter. Here we will see if the PC is getting addressed properly. I would also check to see if the computer has an ip address statically assigned. I would also look under IE settings under the connections tab to see if a proxy server somehow got enabled.
Open command prompt and try to ping www.google.com or any other site and see if you have connection at that level. It could very well be a browser issue.
With this info, you could have 100 different solutions to your issue.