[dg7feq]

It looks like the signal is too low for UMTS data connection.

I would try to set your modem to GSM/GPRS and try to connect there first.
Your APN is correct - internet.eplus.de
username is eplus, password is gprs (i dont know if the settings are the same for UMTS- in our area there is no eplus UMTS coverage to test).

EDIT: I just saw that your modem dials ATDT*##***##
But the dialstring for GPRS/UMTS is *99# (or *99*1# or *99***1# - depending on the hardware)

Chris