I like to do a heads-up about possible data connections by wifi (broadband access).
Hotels, restaurants and cafés often offer wifi access - free or when you buy something.
Most large train stations of Deutsche Bahn and also some cities / towns offer free wifi spots, e. g. Berlin public WLAN.
On top Deutsche Telekom and Vodafone (two main broadband network operators) have thousands of wifi points in Germany which can be accessed by a wifi pass (Telekom) or a Hotspot ticket (Vodafone).
Telekom Hotspot info (German web page) - search for hot spots on the map.
Vodafone Hotspot info (German web page) - search for hot spots on the map.
btw: in Germany ask for WLAN (wireless LAN) if you want to ask for wifi - technically both words are not exactly the same but they are used vice versa.
For mobile data tariffs also Deutsche Telekom and Vodafone are the best tested operators since years (results overview of 2024 test). Currently the mobile network of Deutsche Telekom provides more stable and performant connections, especially on the landside.