If your filtration system consists of only an RO membrane, you will need to install or add on additional filters.
Most RO/DI systems, say with a "4 stage" RO/DI, the water first is filtered by the sediment filter, then carbon filter, then to the RO membrane and finally through the DI resin filter then to your RODI water storage.
If you have all of these and still can't get 0 TDS, then either your filters need replaced OR your source water is dirty enough that you need additional filter units.
Without pre-filtering the incoming water, the RO membrane likely won't last long at all. It is doing the work of 3 other filters.