New Prepaid Plan Not Recognized

I had a prepaid plan that is active until 12 May, 2022. I saw it prompted me to either give a new code or I will be charged regularly for my plan starting on the 12th. I bought my prepaid plan as usual and entered the new code, and saw the green 'accepted' message (it's all in German). But the date has not changed, it says 'give a new code until 12 May or pay regularly', so it looks like I don't have a year's worth of the plan as I should, having entered the new code. 

What can I do?