If you have any problem with TTL setup, i can help.
By the way, i heard positive reviews from D500 users, who installed TTL board.
I don't believe the problem is one of setup, but rather inconsistency of the TTL converter. For example, a series of shots of the same subject yields widely varied results. Some overexposed, some underexposed, some right on. Also, same subject, varying the aperture, one would expect the TTL to do the compensation and provide very similar results. Nope, results all over the place. And for some strange reason, everything works fine on the bench.
Given that everything made falls within tolerances, and that 2 devices that have to work together can be at the extreme, opposite, ends of their tolerances. So it's possible that that is what's happening here. I've considered trying a different camera body to see if that makes a difference. I just don't happen to have a spare D500 body laying around.
So for now, for us, the converter remains unreliable and only used in the manual position.