Talking between Ardunio UNO and LaserGRBL Software

[TH137]

Hello everyone,
I converted an old 3D printer and equipped it with a laser. It's running on an Arduino UNO with a CNC shield and DVR8825 motor driver. I've loaded the LaserGRBL software, and everything is working great; I'm very satisfied.

Now, I want to integrate a second Arduino UNO with a Touch Screen Shield. The plan is to read G-code from an SD card and then send it to the Arduino UNO running the LaserGRBL software. Additionally, there should be a home button on the touchscreen that sends "$H" to move the laser to the home position. I've already programmed the basic framework for this setup. The communication is intended to happen via RX/TX.

Unfortunately, nothing happens when I send "$H" to the laser. Is it possible to modify the LaserGRBL software so that the Arduino expects something on the TX pin? Or is it only possible to control the laser via the USB interface?