According to the help site: "Dual() combines a number and a string into a single record, such that the number representation of the record can be used for sorting and calculation purposes, while the string value can be used for display purposes." Therefore it is limited on selecting the text representation of the DUAL field.
Instead of using selectValues() and selecting on the numeric values of the field like David said, you could use selectMatch() method, and provide the text values you want to selected in a seach syntaxt.
e.g. app.field('DayOfWeek').selectMatch('(Monday|Tuesday|Wednesday)', true);
You can also use toggleSelect() method for toggling your selection.
Please mark this answer to be helpful and close the thread if it works for you.