Hi Steffen,
iFrame UI element has been deprecated on NW EHP1 but its allowed to use with restrictions from NW EHP2,
Please refer the below link
IFrame - Web Dynpro for ABAP - SAP Library
As you are already on NW711, the ui element iFrame should be available in your system.
Just give a try once again.
Creating iframe UI element in webdynpro.
Regards,
Rama