What is the sensor for on the s21 if it doesn't work stand alone? It worked perfectly on my s9+. So I "upgrade" and lose features, as well as no SD slot? I have a gear s3 frontier, but can't get bp, stress or o2 readings with it. I was using My BP Lab, an awesome app and the sensor on my s9+ took very precise measurements. So now what? I have a top of the line phone that does less than my old phone?