For numbers to sort correctly, they must either all have the same number of digits or be entered in a number field.
01, 02, 03 and 12 will sort correctly in a text field.
Since you also have text in this field, you may need to create a hidden text field that adds the needed leading zeroes and then you set your portal to sort on this field instead of the visible field.
Right ( "000" & TextField ; 4 )
will make all entries 4 characters long, padded as needed with from zero to three leading zeroes.