Numbers: Change Date/Time Format on Imported Spreadsheet

Numbers 6.1, MacOS 10.13.6 (High Sierra)

I opened a spreadsheet from a .csv file downloaded from an online database. There is a Date of Birth column that imported dates as text in the US format (mm/dd/yyyy). My system settings are U.K. Custom so that dates show up as dd/mm/yyyy. I want to either tell Numbers either to view the dates in this spreadsheet as mm/dd/yyyy or convert the dates to the dd/mm/yyyy format.

Here's what I've tried so far to no avail:

• Highlighted the column that contains the Date of Birth and changed the Data format to Date & Time, then changed the Date to "5 Jan 2021". Entries for which the middle number was 12 or less converted to dates, but Numbers assumed that the first number was the Day and the middle number was the Month — i.e. 12/04/2016 converted to 12 Apr 2016 instead of 4 Dec 2016. Any entry for which the middle number was greater than 12 just didn't convert — i.e., 01/26/2016 remained as text.
• I tried using the Left, Mid, and Right functions to separate out the parts of the date and then Concatenate to combine the numbers in the correct order — i.e., 01/26/2016 separated out to 01, 26, 2016 in separate columns and then recombined them to 26/01/2016. I then tried to change the Data Format in the column with the recombined numbers (i.e., 26/01/2016) to Date & Time and the Date format to 5 Jan 2021 to make it clear whether or not the format has taken. It did not — the Data Format for the new column remained Automatic and the "date" was still text. I even tried changing the Data Format for the separated values to Number, but the recombined cell still remained text.

Is there a way to change an imported spreadsheet containing dates in the mm/dd/yyyy format so that Numbers correctly converts the dates to dd/mm/yyyy?