Could be a permissions problem - have you joined all of the correct groups (dialout, maybe tty) ?
With LTModems you usually have to go back to the basics to get them to work - check that the module's installed, that all permissions are correct, and that you're pointing at the correct device (use eithe KPPP or Wvdialconf to check which device is active).
Cheers,
mj
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