Have you forgotten what baud rate the device has been configured to? Don’t worry! The device can be reset to factory defaults by tying the RX pin to GND and then powering up the device.
Once powered up you will see alternating - (dashes) and _ (underscores) for 1 second.
Once you see a rotating display of 0-00 you will know the device has been reset to 9600bps. You can now release the RX pin from ground, and the display will continue to function normally.
There is also an example sketch to show you how to do a reset over serial. This can be handy if the display is installed in an application and you can’t pull the RX line low during power up.