I sync with my office Exchange server. This incorporates the external IP Address. You cannot connect to this address from inside the office. For example if the external is 184.108.40.206, inside the LAN there is no need to go here so the DNS server doesn't really know what you're talking aobut. You would go to the class C address of something like 192.168.0.1 inside the LAN. However, doing this would require you to put in one IP address inside the LAN and one when you're outside.
It's much more simple to just use the external and use the 3G connecitivy. Is there some reason why you need to connect to the LAN?