[wolfbln]

Have in mind that getting the Eplus/O2 networks (with free domestic 3G roaming between them) Aldi talk is operating on, is a bit tricky in boarder regions.

As Konstanz is located in the boarder region to Switzerland with very high roaming fees, you should better make a manual network search and either lock it to Eplus or O2 (only if your local 3G coverage is much better on O2, as you won't get any 2G on O2). O2 is recognized as a domestic roaming network of Eplus by many devices. But you can't allow data roaming without allowing Swiss networks at the same time too.

This way you can't jump mistakingly onto a Swiss roaming network with high roaming fees as Switzerland is not a part of the EU.
But you will possibly lose the German domestic O2/Eplus roaming with an extended 3G coverage.
Going north through Germany, it's therefore best to re-activate automatic network selection to get both networks of Eplus and O2.
This is ongoing flaw of the domestic Eplus/O2 roaming in boarder regions, but can be only solved when both networks will be fully integrated in one or two years.