You were pretty much in the right direction, but you were unhappy with your particular shortcut choice. Ctrl+Tab is a common shortcut to change the next open Tab (as Alt+Tab does for open Windows). I'm not sure if this is set within TeXstudio, or at the system level. But apparently, we cannot steal it from its original function (that's not quite true, see update below).
But you can choose another key combination, of course. I have my settings as (of course, what is pertinent here will depend on your keyboard layout):
Previous placeholder = Ctrl+>
Previous placeholder or character = "none"
Previous placeholder or one word left = "none"
Next placeholder = Ctrl+<
Next placeholder or character = "none"
Next placeholder or one word right = "none"
Now, if you, like me, are annoyed by this particular shortcut because it steals the very common behavior of Ctrl+Right/Left to navigate between words in the editor, you might also want to set:
Move cursor left (1 word) = Ctrl+Left
Move cursor right (1 word) = Ctrl+Right
Update: (courtesy of @Troy) As it turns out the shortcut Ctrl+Tab is indeed defined within TeXstudio as the additional shortcut for "Next document" (in "Menus" -> "View"). If you remove this additional shortcut for "Next document", you can then use it for "Next placeholder" instead, as you originally intended.