Can't detect Radeon RX 580 & Akitio Thunder2 (MacBook Pro 2014 13" Retina, Mojave 10.14.3)
When I was first trying to get my Thunder2 working I found that the black metal part that the gpu io bracket screws into was ever so slightly keeping the card from properly going into the PCI slot. The card wasn't being recognized in a similar way to what you've described. Taking out that part fixed the problem (and I eventually bent the io bracket on my gpu to fit with it in but I'm not sure I recommend that).
Here's a picture of the part I mean:
@alex_dubinin, Hey bro! I'm sorry if it's too late already, but may I ask how did you eventually get it working? I think I quite literally have the exact same setup and problem as you (except my card is RX480). Thank you so much ! 😀