ModBus RTU with LWip
if you do not want to write your own modbus RTU implementation you will need a modbus RTU master stack.
Haven't tested this one but you could try it out
https://github.com/stephane/libmodbus/blob/master/src/modbus-rtu.c
Related content
Development tools for Zephyr on STM32
Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.
Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.