Fitbit for Heart Rate Monitoring

My doctor said that I should keep my heart rate below 140 bpm while working out so I bought a Fitbit charge 2 which monitors your heart rate. I'm not sure if it is accurate or not but it's nearly impossible for me to keep it below 140 bpm even while running very slow. I wasn't even breathing hard and it was an avg of 152 today. I'm thinking I might need to try another heart rate monitor. Has anyone had a better experience with this? What did your doctors say?