The third party cellular connection manager software that comes with a cellular devices or is downloaded from the cellular device manufacturer's website can interfere with the normal operation of cellular devices when the AT&T Global Network Client is running. This is normally occurs because the third party connection managers are often configured to auto launch when a cellular devices is inserted. When two cellular connection managers are running at the same time this can cause unpredictable results. You should either configure the third party cellular connection manager so that it does not auto launch on device insertion or disable the cellular connection monitoring in the AT&T Global Network Client through the preferences menu.
Systems the come with embedded cellular devices will almost always come with their own connection manager software. It is typical that these applications are configured to auto start. To use the cellular functionality in AT&T Global Network Client you should set the OEM's connection manager to not launch when Windows starts.
HP Connection Manager - start the HP Connection Manager with the SIM in the machine. Select the Options menu item and then choose Application Preferences item. On the Application Preferences dialog unselect the "Launch application when Windows starts" and press OK. Finally, select Exit from the File menu. The name of the HP Connection Manager process is gbxApp.exe.
Sierra Watcher - start the Watcher with a SIM in the Sierra device and select the Tools menu item and select the Options item from the list. In the User Options dialog in the Startup section set the "Autolaunch on device insertion" to "no" and press OK. Close the Watcher to use the AT&T Global Network Client. The name of the Sierra Watcher process is Watcher.exe.