You could replace the radio buttons by check boxes as well. This would give your users the desired ability and enable them to select more than one class. If finally no users were selected, then the Post button should be inactive.
The default component for single value selection (same behaviour as a radio button essentially) is the picker: https://developer.apple.com/design/human-interface-guidelines/ios/controls/pickers/
You can use that to enforce single-select behaviour on tablet and keep the default checkbox behaviour that iOS provides for multiple selection.
Alternatively, you ...
A little bit of column A and a little bit of column B.
Standards are difficult to change especially when you have to work with multiple companies and browsers. With every change, you have to ensure browser compatibility for all available versions for all available browsers, you have to consider accessibility, touch behaviour etc.
As technology advances (...