Hi again,
I'd like to share again my experience which is a possible fix (I'm on 3G for more than 14 hours now, which is a new record), and which is related to Bfreeman1969's post.
First of all here is my configuration of SIM Cards, and I feel that the problem is related somewhere with that:
1) One "Master" SIM Card in my iphone 5s
2) One "Slave" SIM Card in my iPad 2 (Vodafone Germany, it is called Ultracard)
3) I have the setting Settings --> Facetime --> iphone cellular calls enabled.
The Ultracard in the iPad is assigned to the same cell phone number as the master-SIM in the iPhone, that means any cell call is routed to both cards by the network provider simultanously. Strange thing happening: If someone calls me on my cell number, I found that the iPad is waking up for a few seconds (showing the lock screen and doing nothing else), then going to sleep again. Afterwards the ipad will show the missed call.
And no, the devices are not in the same WiFi Network, they are 5 km away from each other! I tested this putting the iphone in fligth mode next to the ipad, this thing is happening as well.
This made me disable the settings for iphone cell calls on my ipad (Settings--> Facetime --> iphone cellular calls set to disable)
Now the ipad is no longer waking up when someone calls me, and as a side effect, it remains on 3G and the problem with losing mobile data network seems to be solved.