@justGuner Sounds like your CAN-FD board may have died then. Not something I've seen any other reports about. Probably worth examining the CAN-FD board to see if you can see any sign of damage or anything shorting any of the pins.
It seems very odd that the maiinboard seems to start up and only reboots after a short while. If you unplug everything else from the board and just have the CAN-FD connected do you seem the same issue? When you are running these tests how are you powering the board?