Make sure you use the proper cuff size for accurate readings. Also in my experience the wrist blood pressure monitor does not give accurate readings. Avoid that kind if you can.
Devices like this are regulated, so it should be reliable. Keep in mind that normal BP does fluctuate through the day especially with some medications, so you can't really rely on any single reading but rather on a daily/weekly/monthly average.
Also in my experience the wrist blood pressure monitor does not give accurate readings. Avoid that kind if you can.