The project has been included to give as much information as possible. Nobody is obliged to download it.
The question is the following: if I have that the ethernet works regularly at 64Mhz of cores, and I only change the frequency of the core to 400Mhz, is it normal that the ethernet no longer works?
Perhaps ditch that example software, grab the latest STM32CubeIDE and spin yourself a clean project for your board and start from scratch.
Read community posts daily. That'll teach you what people using the tools or your board/core are struggling with and what works/what doesn't.
Start with something simpler than ethernet your app needs to do, conquer it, then tackle ethernet again (without looking at that bad example).
If you run into trouble, post a concise description to this community what you're using (cube version, h7_fw version, board type, etc, etc), what you're trying to do and what you're observing, and include a few paragraphs from the RM you're reading, and you'll receive good answers fast --- because you'll know more what you're doing and so you'll be able to explain it, and people here will understand it because they're using the same tools and junk too.
