Finally, success in getting 3G in Plasma Active on Nexus 7 to work. :D I uploaded a patched version of ModemManager to the Nexus 7 hardware-adaptation repository. So far, it seems to work pretty reliable.
Please note that I disabled the pin on my sim card. I don’t know if this is necessary but it was at least one point less I had to worry about. You can find instructions on how to disable the pin here (I got these instructions from this web site.).
For those of you who like a little bit more detailed information: the trick is to replace the dial command “ATD…” with “AT+CGDATA=\”PPP\””. You can find the patch in the patched ModemManager package in the Nexus 7 hardware-adaptation project.