Probably you won't be interested in domestic airtime but Fonic subscribers can now subscribe to all those bundles (and pay with their prepaid credit) that required direct debit payment through a German bank account before:
https://www.fonic.de/fonic-tarife-ha...biles-internet
In contrast to Fonic, Lidl Mobile (which legally is also run by Fonic) still requires all those "smart" options (bundles of airtime and data volume, NOT mere data bundles) to be paid by direct debit and so are not an option for visitors.