Im pretty sure you have not knackrered your Membrane, OK
Chlor will ruin the Membrane (Thats why you have the Pre-Filters)
but if you have only done this for a short time you wont have done any damage (IMO).
The Backwash is done do remove build up on the walls of the Membrane housing (Inside of course), what happens by the Backwash is that more water passes out through the side of the Membrane than throught the Membrane itself therefore washing out all sticking sediments, this goes straight to the drain, to do this your system must have a valve system to adjust the amount of pure water passing through and the amount being rejected, the backwash procedure wont produce much RO water but flush the Membrane. Check with your supplier how the intended to Backwash.
If you need to replace the Membrane its going to be quite expensive and difficult as the Ends of the Membrane are basically sealed and very difficult to remove and need resealing 100% tight fit. Ask your supplier if it becomes necessary whether he will do this for you (Warranty!!).
I reckon if you find out how to Backwash you will only need to do a longer Backwash (maybe an Hour) and your TDS values will improve again.