RS232 is the voltage level, RX,TX is the serial communication, these are two concepts.
For example, suppose you are using an MCU whose level is RS232, but your LCD level is TTL, in order to have normal communication between LCD and MCU, you have to make sure the LCD and MCU level and baud rate are the same. To do this, you will need to solder the resistors or remove them as needed at J17 on the back of the LCD. As shown in the figure below. UART serial communication is information sending and receiving, you just need to connect the TX of the MCU to the DIN of the LCD, RX to DOUT, and GND to GND.
The adapter board is used when the LCD is communicating with the computer.
