Under best case scenarios, a signal booster is able to provide advertise coverage. However, for the majority of people, most are not under best conditions.

Several factors such as outside signal strength, building material, internal interference, and even weather can affect the boosted signal coverage.

It's best to learn about dB readings and understanding signal strength in numerical values than bars: https://www.wilsonamplifiers.com/blog/db-gain-the-most-important-thing-to-your-cell-phone/