Looks like an HM DM-1 dual TDS meter which is what most assemblers of RODI systems offer. They're factory calibrated with NaCl solution, so should be good out of the box. I would however be somewhat suspicious of a 0 TDS reading after the RO membrane particularly right after starting the system up. My tap TDS is around 184 & it takes a few minutes after start up for it to settle down to 1 TDS after RO.
Sediment filters do little or nothing to lower TDS, they're designed to trap solids, not dissolved solids. I installed a second TDS meter on my system to monitor the tap water TDS vs TDS after my two sediment filters & the carbon stage. Shortly after I put pressure to the system, the TDS after those 3 stages (before the RO) is actually higher than the tap water reading to the tune of up to +20. I presume this is due to stuff dissolving off the sediment filters during the days the system sits idle. Then when I put pressure to it, that stuff gets sent to the RO membrane. Takes a good 5 minutes for the reading after the pre-stages to drop below tap water TDS & it's usually only 1 or 2 TDS difference.
Might be worthwhile for you to double check where the probes for your meter are plumbed in. As a quick confirmation that it's working, you should also be able to pop both probes out of the JG fitting & stick them in tap water to see what you get for a reading. This will tell you how well it's calibrated, if there's a significant difference in readings between the two probes in tap water, you know you have an issue. And if you do remove the probes from the fittings, make sure you align them properly when you re install.
More info at their web site:
http://www.tdsmeter.com/products/dm1.html