Not only large series dispersion ...

In terms of hardware, it seems to be quite a large series dispersion in this FB. The first FB6842 could Although the LTE network (Munich city area 1800) login, but immediately lost in traffic the connection. The FritzBox builds this connection while automatically back up, but this behavior was not only extremely annoying, no reasonable data exchange, it was also possible.
So FB returned and worried exchange.
With the same SIM card (Telekom mobile data contract) turn easily the LTE can build connection. And this time, the data transfer works perfectly and permanently, no disconnections, and even quite good transfer rates (up to 20 MB download and upload 6MB).
All other features of the FB can be configured as usual with AVM very user-friendly, which is good like the alignment using the internal LTE antennas of the box. This allows the best location for the box depending on the space available, see very easily.
The additional software features of the box are as always very extensive and extremely convenient to set up, so there is absolutely nothing to complain about except for the malfunctioning first box.
My tip for similar problems: not long hesitate, but replace FB.

UPDATE 30/10/2013
Have the FB6842 LTE now run with a telecom data plan in continuous operation and experiencing lately a significantly increasing number of disconnections. It falls in terms of disconnections of a serious on:
Is the FB 6842 LTE via LAN integrated network with 3 PC's and is demensprechend "required", we have some days 2-3 disconnections per hour!
If all PC's only via Wi-Fi connected to the FB 6842 LTE, there is significantly less breakage - only about 1 abortion per hour. Mostly the way in off-peak times when the FB is "required" by any PC.

A real explanation for this phenomenon, we do not currently have, the site of the FB is unchanged, only the Kabelei is correspondingly reduced (possibly a hardware problem ??).
Nevertheless, the FB immediately dials again - and discontinuations during large data transfers also are very rare - are these discontinuations during low utilization of course enormously annoying.
We will consult the AVM service it.
Currently I prefer in my review but a Star or More ...
But we observe wieter ...

UPDATE 01.11.2013
Have now tested as a whole working day to another LTE modem (Huawei e589u-12) operated at the same location with the same telecom LTE-capable SIM card. Connected with 2 PC's + 1 Smartphone via WLAN.
Result: only in 8 hours NOT disconnect, no waiting for traffic (download as upload) - a ratzfatz Internet connection (an average of 35 Mbit / s upload and 8.5 Mbps download) even under continuous load.

This is clear to us that the issue of "permanent disconnections" Needless to be related causally with the Fritz box.
Therefore now and here again 1 star deduction ...
Sorry!