Aldi is a well-known German supermarket (not only in Germany) and they sell SIM cards which operate on the Medion Mobile / E-Plus mobile network.
The Pay-as-you-go rates are pretty cheap (better than NettoKom's, the cards cost (in 2019) €13 with a €10 credit already on them, but you have no idea what the mobile number is until after you buy the card and open the package.
Any credit lasts up to 12 months, and when you top up, the time the top-up lasts for depends on the value of the top-up. To keep the card going for a further 12 months costs €10, so NettoKom is a better bet if you mainly want incoming services only.
alternatively:
and then enter the 16-digit code when prompted.