Ok, good news is that on server side your settings are 100% correct. DTR/RTS have no meaning for pure serial connection.

This response: Recv:3:57:05.764: echo:Unknown command: "[ 17.598172] rc.local[1354]: My IP address is 192.168.1.52"

shows clearly there is some other software talking via serial which interferes the communication and causes lots of issues. My guess is that linux is configured to also output everything to serial console.

To disable the serial console on a Raspberry Pi, you need to modify the boot configuration to remove the serial console settings and potentially disable the serial getty service. This prevents the kernel and other processes from using the serial port for outputting messages. <div><div><div><div>

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Here's a breakdown of the process:<div><div><div><div>

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1. Modify /boot/firmware/cmdline.txt:<div><div><div><div>

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Open the cmdline.txt file in the /boot directory using a text editor with root privileges (e.g., sudo nano /boot/cmdline.txt).

Locate and remove any lines containing console=serial0,115200 (or similar, like console=ttyAMA0 or console=ttyS0). These lines specify that the serial port should be used as a console.

Save the changes and exit the editor. <div><div><div><div>

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2. Disable the serial getty service (optional but recommended): <div><div><div><div>

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The serial-getty@ttyAMA0.service (or similar, like serial-getty@serial0.service) is responsible for starting a login process on the serial port. To fully disable the console, you should mask this service:

sudo systemctl stop serial-getty@ttyAMA0.service

sudo systemctl mask serial-getty@ttyAMA0.service

You may need to adjust the service name (ttyAMA0 or serial0) depending on your Raspberry Pi model and configuration.

What image are you using? pi default is to have it but I thought I had removed that.