I thought it was your thread here
Germany - Tchibo - Prepaid Wireless Internet Access, from which I copied the above USSD commands.
Anyway, returning to my question is it possible to activate "Internet Flatrate XL" by using USSD commands in MDMA (if yes, what are the commands) ?
Regarding your suggested text message, I must to confess that I have no idea of how to send it. Sorry, you only tought me to send USSD commands in MDMA and to receive SMS by using Mobile Partner. Is sending text message just the other way around ? Unfortunately, I can't experiment by myself as my account is still empty and I can't therefore activate it.