I complained to the shop (TK Point). They are official sales partner/reseller of congstar, so congstar is not blameless. In fact, they share the larger blame for failure of maintaining quality of their official partners. The shop person just says they can't do anything. So apart from filing official complaint to some German govt agency, I have no recourse left.
Congstar support says they are looking into it; so I'll wait awhile.
Yes, you are right. I checked out voda and they are giving 200GB (3x of Congstar) + smartwatch support for similar price + $200 refund.
I have 4x Congstar sims in family now. I will switch all of them, just out of spite, if they don't make it whole for me.
Happy new year!