Can you connect to the modem's control panel at all?
The address should be:
http://192.168.100.1/
User Name: root
Password: root
If you can that should rule out a problem with the connection between PC and modem.
Have you checked that any firewalls you are running aren't getting in the way?
Have you tried using a different ethernet lead (I'm assuming that you are using the one that came with the modem)?
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