Thanks for your replies.

@ qwazix: I did a reflash via flasher tool to stock PR1.3. In first step, with the stock version of PR1.3, Nokia Link reappeared but there was no 3G connection possible, only 2G internet connection.

But then I made the Nokia Link update and: voila! My 3G connection works perfect now, the Nokia N9 switches perfectly from 3G/3.5G to 2.5G if needed and the speed of browser and other applications is quite fast.

So: the Nokia Link update did really solve all the 3G issues!

Thank you fo your help!!