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 Signal Spy. The purpose of Google Fi is to switch cell phone carriers and Signal Spy provides a way to manually change carriers within the limitations of Google Fi and Android.
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 Signal Spy but of your phone and Google Fi.
Third, the answer to the original question depends on whether or not you have purchased the Pro version of Signal Spy.
If you have the Basic version...
- When you select a dialer code, it is copied to your phone's clipboard. When the dialer opens, you then need to "long-press" in the area where you normally would type a phone number and select "Paste" to enter the dialer code.
If you have the Pro version...
- Accessibility is most likely disabled. Go to your Android settings > Accessibility, and turn it on. Now when you select a dialer code, it should automatically be pasted in your phone's dialer.