The at sign (@) on the German keyboard for the iPad Pro 12.9" with the Belkin Pro Keyboard is usually entered using the key combination "Alt" (Option) + "L". You hold down the Alt key and then press the L key to create the @ sign. This combination corresponds to the standard assignment on German keyboards for iPad hardware keyboards.

If this does not work, you should check in the iPad settings under "General" > "Keyboard" > "Hardware keyboard" whether the keyboard is set to German and whether the special key assignment is correct, as incorrect settings (e.g. "Option" as a globe key) can affect the input.