The failing subscriptions of data options by SMS are a known persisting issue with Tchibo. In that case you can only order by phone or maybe by USSD, however I don't know for sure if that works.
Before retrying to reach an English-speaking agent at customer care, you could try the USSD-method:
Just enter *104# into your phone and await a menu to be displayed, then there should be choice of options, where one is for ordering data options.
If you have your Tchibo SIM in a USB-modem, you can send that USSD-command with a tiny tool called
MDMA.
If you try the USSD-method please let me know, what exactly is displayed and which numbers you need to chose for the data options, so I can ammend this info to the
PWIA wiki.