Welcome to Android Central! When you say "navigation bar", do you mean the status bar at the top of the screen? The navigation bar would be at the bottom, where the navigation buttons are (if you're not using gesture navigation).
If you are talking about an E icon on the right of the status bar at the top of the screen, that usually means the phone has dropped down to 2G signal (EDGE, which has been shut down in the US and in many other parts of the world as well). Therefore, you either have no 3G/4G/5G signal in the area, or your phone doesn't support those network bands on the carrier you're using.