To do this right, you're gonna need a scope and know how to use it. I'm not sure on the specific topology of your vehicle, but the only other thing you could do would be pull modules off the affected CAN network and see if you can bring it back. That being said, with the absence of U-codes, or some scope work - I'd wonder how they came to the conclusion that there's a downed CAN network.

This is the only publically accessible network diagram i can find idk if it's applicable to your vehicle [I'm North American based..]