Congstar - cheapest data prepaid in Germany?

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buy a starter package in any shop listed on their website. They have packs with SIM only, together with a cheap phone or a cheap USB data stick.

register the card on www.congstar.de/freischalten
a help video is here
congstar Prepaid

log on to the customer service page and follow the instructions to book a package:
[HOW TO] Postpaid Optionen (z.B. Surf Flat Option 500, SMS Flat Option) hinzubuchen - Postpaid - congstar Support Forum

unfortunately all pages and info is in german, but maybe someone local can help you.

Without german bank account you can top up by buying the scratch cards in any shop that sells Congstar cards (Or use original T-Mobile Xtra cash-cards that are sold nearly everywhere). There is a store locator online:
Aufladefinder.congstar.de