Please note the 1st bullet point at the beginning of the Support Article to which you provided the link... It's now clear that you are not using a
U.S. Keyboard Layout, so it stands to reason that the same combinations will not work for you.
That also explains why you have no +/= key... The German keyboard doesn't have one. On a U.S. keyboard that key is used for both sub & super: whether the Shift key is used to trigger upper register is the determining factor. I've never used
a German layout before but you might try these combinations:
Superscript: Command+✚ [using the key to the right of Ü]
Subscript: Command+Shift+0 [the upper register of the zero key which is the equivalent of Command+〓]
If your installation is German perhaps using Help> Word Help to search for
keyboard shortcuts will provide a list of those based on a German keyboard.
Another option is to go to the very bottom of the Article you cited & select German as the Language. I'm not sure of the translation but I believe it's indicating exactly what I wrote above:
