Work-around for missing Text function in iOS Numbers
Seems there is still no TEXT function in Numbers in iOS.
There are of course the two excellent work-arounds suggested by Barry (March 2020: format hidden cell, and concatenation).
I have also found that I can format the cell in Microsoft Excel, then copy that cell into Numbers which will display the formatting chosen in Excel - not sure if that works for all formatting but it seems to do the trick for dates :)
iPad Pro