How long has it been when you change out the media? Mine has been going up as the media filter and rodi getting spent. My tap is sometimes as high as 500-600+ with 0 reading but lately its been around 350-400.
I replace the sediment and carbon block every 6 months. I only change the DI resin when the TDS is 1-2.
Keep in mind the prefilters protect the membrane from damage caused by sediment and chlorine/chloramines. The DI is used to polish the post RO to 0, so the two scenarios are not related. Keep the membrane protected or it will be costly.