@lina @MK2boogaloo @syzygy if the GPU doesn't have a video signal even while booting, it's not a driver problem. Either the GPU is fucked, or the Motherboard slot is fucked.
But he already tried with another MB, and the other GPU works fine, so this GPU might be fucked.

What I would try would be another PSU. Sure, the GPU may not need a six pin power connector, but maybe the PSU is not delivering enough power for the RX560, but is delivering enough power for the other GPU.

If that also doesn't work, then I'm sorry, but the 560 is dead.