There isn’t any way in ACF to identify rows of a repeater. Rows can be moved so the index is not suitable. You would have to build your own mechanism for identifying them. For example you could add another field to each row and dynamically populate the value of this row with some unique value that will never change.
There have been many topics here about doing the above, here is one example https://support.advancedcustomfields.com/forums/topic/unique-id-for-repeater-field/
https://github.com/philipnewcomer/ACF-Unique-ID-Field
What you are trying to do would be better suited to using a custom post type than an repeater on an options page.
