I’m not an engineer, but I fought this exact error for a week with a Pi 3 and a heat meter in our hotel’s boiler room, and it was two things together.
On a Pi 3 the proper UART (PL011) is wired to Bluetooth and the GPIO pins get the mini UART, ttyS0. The mini UART’s baud rate follows the core clock, so when the CPU changes speed the 9600 isn’t 9600 any more. The mini UART also has no parity, and your meter is on 8E1. So the meter hears your request, answers, and the Pi can’t make sense of the reply.
Second one got me: the serial login console. If cmdline.txt still has console=serial0,115200, the Pi writes its login prompt onto the RS-485 bus at boot and keeps a getty on the port.
What fixed it for me:
# /boot/firmware/config.txt (or /boot/config.txt on older images)
enable_uart=1
dtoverlay=disable-bt
sudo systemctl disable hciuart
# raspi-config: Interface Options > Serial Port
# login shell over serial: No, serial port hardware: YesReboot, then use /dev/serial0 (it points to ttyAMA0 after this) in pymodbus. Also check whether your meter counts registers from 0 or 1. Schneider manuals list them 1-based, so 3028 in the book is 3027 on the wire, and you had that right.