STM provides an Ethernet library for Arduino_Core_STM32, but it supports only LAN8742A.
With a little change, You can use LAN8720 PHY with STM32F407.

Note
STM is currently rebuilding external PHY Ethernet support for Arduino_Core_STM32. In the future, Ethernet support using an external PHY will change significantly and may be supported differently. This is an interim action until officially supported.
When STM become officially supports external PHY Ethernet, I will delete this repository.

STM32F407VET6/VGT6 Development Board. About $10 for aliexpress/eBay.
LAN8720 PHY module. About $2 for aliexpress/eBay.
STM32 NUCLEO Board. Because we need STLink V2.1.

STM32 core support for Arduino.
https://github.com/stm32duino/Arduino_Core_STM32

Arduino library to support Ethernet for STM32 based board.
https://github.com/stm32duino/STM32Ethernet

Lightweight TCP/IP stack (LwIP) is a small independent implementation of the TCP/IP protocol suite.
https://github.com/stm32duino/LwIP

You have to update your local file.
File location is Documents\ArduinoData\packages\STM32\hardware\stm32{Version}\system\STM32F4xx.

stm32f4xx_hal_conf_default.h

/* Section 2: PHY configuration section */
#if !defined  (LAN8742A_PHY_ADDRESS)
/* LAN8742A PHY Address*/
#define LAN8742A_PHY_ADDRESS            0x00U
#endif

STM32 NUCLEO provides a on-board STLINK-V2-1 USB interface. Actually the board has two STM32 chips. One is the target of the demo board, and the other is the STLINK. This STLINK-V2-1 can be used as programmer and debugger for external targets. Remove the two jumpers from CN2, and place them at the two outermost empty jumper pins marked CN11 and CN12.

Connect the NUCLEO SWD port to the STM32F407.

Choose Upload method "STM32CubeProgrammer(SWD)"

If another BootLoader has already been written to STM32, connection of nSRTS pin and RESET pin is required.

You need to use a USB-TTL converter to display the serial output.