I don't think sets will work for your use case because you're trying to calculate the top Carrier within each region, as apposed to just the top carriers in an overall dataset.
Have you taken a look at these instructions? Finding the Top N Within a Category | Tableau Software
You can use Rank(Sum([Download_Speed])), the advanced compute settings are at the level of Operator repeating every Area. Then use that as a filter and set to 1.
The solution i can think about it is to create rank field on your selected measure and add it to the level of details, let it compute using operator dimension.
Then add filter on your new field and select 1 which is the top one.
I hope it is clear and can do the trick.
Yes, Tom... I have looked into that topic yet I still can't find any idea... But thanks anyway
Yes, it is the correct list, Krishna... It should be like that... and recently I found out a solution from my friend pretty much like yours...
Thanks for helping.