Check the serial ports

ls -l /dev/ser*

Download the uart-ctsrts dtoverlay file. make sure to change your
location in the directory to /boot/overlays that way the file will be copied
into that directory.

cd /boot/overlays (then enter wget command)

sudo wget https://github.com/HiassofT/AtariSIO/...

Edit config.txt in the /boot directory

sudo nano /boot/config.txt

add these lines at the bottom of the file.

dtoverlay=miniuart-bt
dtoverlay=uart-ctsrts
enable_uart=1

Edit cmdline.txt and remove any console reference only.
sudo nano /boot/cmdline.txt