Hi @redcat
If you want to query a certain post based on the repeater field value, kindly check this page: https://www.advancedcustomfields.com/resources/query-posts-custom-fields/.
But I suggest you set the country as a taxonomy or a new custom field outside of the repeater instead and create posts for each Nationality and Country combination. So each post will only have 20 rows of the repeater and it will be easier to query the data.
I hope this makes sense 🙂