SPI problems over long cable

[nadavzil1]

Hello,
I am using a cutom made PCB in a product my company develops, which was reverse engineer based on FLIR's breakoutboard. when I connect this PCB to a raspberry pi 4 using a shitty connector with some wires I get a perfect streaming of thermal image!
When we moved forward and manufactured a professional cable which is 2.5 m long, suddenly I get the known red square issue. I can still command a FFC though.
I read a lot about the issue and it seems the problem has something to do with the SPI clock and the high cable impedance. I tried to change the value in SPI.cpp line 8 from 10Mhz to 20 Mhz,1Mhz, 100kHz... nothing works. Do I need to do something else?
Or maybe the issue is something completely different, its possible

I'd love any help or idea! I am a mechanical engineer and completely not familiar with this field.

Thank you,
Nadav