Standby / Sleep Mode issue - EM8805


I really hope you can help me with my challenging problem.

First I want say I know that some here might say Dell is responsible to find a solution. But as you all maybe know, support of hardware manufacturers is quite useless for 2nd level issues.

So here the whole story:

We want to deploy hundreds of Dell Venue 11 Pro devices in our company group. These Tablets all contain a Sierra Wireless AirPrime EM8805 4G module. (Dell device name: DW5570e)

For deployment, we have prepared a Windows 8.1 system image on one Tablet device. This system image is based on the original image the Tablet comes from Dell and is just modified for our needs (Additional business software installation, pre-settings etc.)
When I press the power button on the basic development Tablet, it smoothly goes to Standby. (Windows Sleep mode)

The problem starts, as soon I install the Windows 8.1 system image on a second Tablet device.
Here the standby mode is blocked - If i press the power button, nothing happens.

There is a Windows command line tool (powercfg.exe -a), on the affected Tablet device it says:
Standby (Connected)
The following hardware does not fulfill the requirements for this standby state:
Mobile broadband

As this message says, the sleep mode "Standby (Connected) is blocked by the Sierra Wireless module. - As soon I disable the module (in device manager or network connection manager), this sleep mode also works on the destination Tablet.

Please have a look on the 2 screenshots attached, I think it describes the issue very well.

I guess the problem may be caused by an device driver or firmware issue.
Unfortunatelly Dell provides only one device driver update on their website. This driver even causes on the basic development device, that sleep mode will not work anymore!! (Where it works fine if I install my base image back again!)

Another thing I noticed:
If i dismount the Sierra Wireless module from the working Tablet and put it in the destination Tablet (non working before), sleep mode suddenly works there. - Question: Is there maybe a dependency between the device driver installed once and the module/firmware itself?

I have absolutely no more idea how to solve this issue and hope you can help.
It would be also in your benefit if you can provide a solution. - As I mentioned in the beginning we want to deploy MANY TABLETS, all powered by the same Sierra Wireless device.

Maybe you can provide me some (Beta)drivers or (Beta)firmware to test or you have any idea for Windows registry settings, driver cleanup,…

Things I have tried out already:
Installing latest device driver from Dell Website
Installing latest BIOS Update for the Tablet device
Resetting BIOS to default settings
Inserting verious SIM cards and try without SIM card
Installing EM8805 drivers from other hardware manufacturers - with less success (not compatible)
Test edits and removes of many Windows Registry Keys found by searching for “Sierra” and “Dell Wireless” in Registry editor.

One more thing, the Sierra Wireless device works generally very well - It’s only the issue with the sleep mode.

Here some general information about the sleep mode “Standby (Connected)”. It seems to be a quite new feature introduced with Windows 8.1. - This fact increases my thinking about a driver/firmware caused issue:

Thank you very much for your help!