This bug has been reported before, but has survived a couple of cube version bumps.
STM32CubeMX 4.20.1 and f7_cube 1.6.1
in stm32f7xx_hal_conf.h it defines the PHY address incorrectly
#define LAN8742A_PHY_ADDRESS 1
#define LAN8742A_PHY_ADDRESS 0
A symptom of this bug is HAL_ETH_ReadPHYRegister returning 0xFFFF for literally everything, and your cube-generated ethernet code won't work.