Mobile presence sensor unreliable and not working at times

It is not an option if you are using the new app, “SmartThings ( Samsung connect)”.

It is an option if you are using the old app, SmartThings classic, but we have been told that that app will be going away eventually, so you might not want to set that up.

SPECIFIC differences between "SmartThings Classic" (the V2 app) and “SmartThings (Samsung Connect)” (the V3 app)

Meanwhile, the following FAQ on presence might be of interest. ( The topic title is a clickable link)