Lebara Internet APN-Einstellungen für iOS 8

Check that Cellular Data is turned on.
Tap Cellular Data Network. 
(If this setting does not appear on the device, it has not unlocked and settings cannot be manipulated manually.)

Go to Cellular Data section and enter below information

APN: internet.t-d1.de
Username:
Password:

Go to LTE Setup (Optional) section and enter below information

APN: (Leave Blank)
Username: (Leave Blank)
Password: (Leave Blank)