Go to SETTINGS > DISPLAY > AUTO-ROTATE SCREEN. ON will allow screen to rotate, OFF will restrict phone display in portrait mode. You can also pull down the notification bar and look in the shortcut menu for the screen icon that allows you to toggle that feature on and off.
If you need any further assistance with this question, please contact the Sonim Customer Experience Center at 1-833-MY-SONIM (1-833-697-6646) or email us at [email protected]. We are open 24 hours a day, Monday-Friday and we will further assist in troubleshooting.