The STM32F4 has 4 Kbytes of backup SRAM.
They seems not to be accesable at a certain adress
The HAL lib has also a driver for it. But the functions are not enabled default.
The library stm32f4xx_hal_sram is needed for the functions. There are even examples in the example directory.
But how to tell CubeMX to include these libraries, i can include myself but they are gone when a new project is generated again.