Had exactly the same problem with my Studio 15.
Solution: Uninstalled Control Point and replaced it with Dell Wirelless Manager Version 5.2.1045.43 - it works with my ATT WWAN flawlessly. I got the ftp link to doanload it off of a forum but could not locate it for you. You might call dell and ask them where to find it.
If you don't have any luck I can try to email you the install file. <Admin note:Email Id removed due to Tos violation>