Oh, I a remember this is the target which is supported just with GCC, so it is usable just with offline tools under regular Mbed.
NUCLEO-H723ZG support - Mbed OS - Arm Mbed OS support forum
Unfortunetely I do not have this one so I can not test it.
Maybe you can compare this init file with generated file from CubeIDE mbed-os/connectivity/drivers/emac/TARGET_STM/TARGET_STM32H7/TARGET_NUCLEO_H723ZG/stm32h7_eth_init.c
BR, Jan