User having trouble connecting to VPN over mobile broadband

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I've got a user with a laptop running Windows 7 who is trying to use an AT&
T mobile broadband card to connect to a VPN using the Cisco IPSec VPN clien
t. The card is the Sierra Wireless Momentum 4G. The VPN connection establis
hed fine, but no traffic gets passed after that. Other users can connect to
 the VPN fine (not using mobile broadband, though). Wired and wireless conn
ections from this laptop are able to get to the VPN fine. It's just over th
e mobile broadband service that this happens. Are there any workarounds for
 this issue?

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