This is bit tricky logic.
Every time you move from 1 page to other, the data will get loaded freshly.
Option 1
Save the selected value data of DDL on every page navigation in to the DB.
Onbind you check the saved value and change the selected index of the DDL.
Option 2
Store the selected value in some temporary place and re-use(But home values you can save? If the pages more?)
Please ref Brij Ans