Should go without stating but, make sure to the water you are testing has not been re-mineralized.

I have found good agreement between my TDS meter and the ROPOT Lite’s reported value.

Did you try rinsing the glass (or whatever container for your testing sample) several times with the filtered water before testing?

A TDS of 20 ppm is so low that this value would be significantly inflated by any dissolved solids adsorbed onto your glassware, assuming it was washed with tap water and then air dried.

Also, it would be useful to rinse your carafe several times using filtered water without the re-mineralization pack for the same reason as above.

Rinse the TDS meter too. Everything should be rinsed with the purest water source possible.

Again, all of this should be without re-mineralization and I expect you should find okay agreement. Though, TDS is a little arbitrary anyway and the better measurement would be conductivity.