Using a broken TDS meter

I have a an RO system at home which produces clean water however. When I purchased the system I also purchased a TDS meter and some NaCl water to calibrate. This was about 6 months ago and when I use the TDS meter with the NaCl water it doesn’t show the right number and there is no screw on the back of the TDS meter. Even though the TDS meter isn’t calibrated can it somehow be used? Or should I buy a new one?
Is there any other way to test if the water my RO system produces is clean?

buy a new one they are

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