Ah, so you're talking about these guys I guess http://www.three.co.uk/Mobile_Broadband
I don't know anything about those dongles or what restrictions they would have (other than the general NAT cell phone warnings I mentioned earlier).
I wonder how you got SC to work -- were you able to put the "viewer" into listening mode and successfully access it by its public IP address?
I just don't know that your issue has ANYTHING to do with the dongles -- perhaps you can test your setup using more common hardwired broadband or office or cafe Internet connections? The idea would be to identify or eliminate the dongles themselves as an issue.