Hi Stef:
Thanks for choosing the STONE product. We have some model available directly working with TTL level. You can check the backside PCB of the display, and see if you can find J17 jumper.
If you order the display on Amazon/eBay, normally it is set as the RS232 level. You can change it to TTL level by short connect the jumper.
If you are not sure what to do, please share a picture of the PCB of the display so I can help you.