1. First of all, do you have Google Fi? Without Google Fi, the dialer codes will not be able to change your cell phone carrier. This is not a limitation of Fialer. The purpose of Google Fi is to switch cell phone carriers and Fialer provides a way to manually change carriers within the limitations of Google Fi and Android.
2. Second, do you have a "designed for Fi" phone? (Find out here.) If not, you will not be able to manually switch carriers. This is not a limitation of Fialer but of your phone and Google Fi.
3. Is Fialer set as your default dialer? Fialer must be set as your phone's default dialer in order to work. Read more HERE.