It seems you are missing the critical part of converting RS232 signal (which is not 3.3 V) to RPI GPIO compatible 3.3V Signal as described in this example: https://www.abelectronics.co.uk/kb/article/20/raspberry-pi-serial-port-usage http://raspberrypi.tomasgreno.cz/uart-to-rs-232.html
You can either create your own converter like this people did: http://justanotherlanguage.org/content/jallib/tutorials/tutorial_serial_port_board
Or buy a converter designed for such purpose: https://cablematic.com/en/products/adapter-raspberry-pi-gpio-txrx-to-rs232-db9-male-MS050/
Good luck