Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32f7/STM32F746/Ethernet_MMC/MMCTGFMSCCR#0x0
Ethernet MMC transmitted good frames after more than a single collision
TGFMSCC
https://github.com/modm-io/cmsis-svd-stm32