Vodafone somehow is/was blocking new updates due to some VoLTE (VoWiFi ?) related tests they seem to do first. But frankly I thought that situation changed a few months ago.
There are workarounds: If you have a SIM card from another provider - also PrePaid sim cards - you could remove your 1&1 sim temporary and insert a sim card form a different provider.
You can also update your FP3 manually with a computer. But you have to unlock the bootloader and thus loose all your data on your FP3. Therefore a backup is highly recommended 
EDIT: Or you wait a few days more until Android 10 is released for FP3. Maybe you get this update more instantaneously 