Unicode HEX Input on iPad Pro

work around is to copy paste from a webpage

or to have the chars in the notes app or a text file in icloud drive or googles cloud app or ms's or dropbox

or other places one can make the chars and easy copy paste them to the mail app

There's no setting for what you want. Use an app like Unicode Pad to get the fraction characters you need and then set up text shortcuts to generate them.