So are the fans not spinning at all then or are they just spinning slowly? If they spin, but don't kick in under load like you would prefer, then if you've tried replacement fans, maybe try a program like MSI afterburner, and set a custom fan curve that at specific temperatures you specify the fans will kick in.
I have a Sapphire brand RX 480 Nitro+ 8gb. Unfortunately, when I first got the card, it liked to get hotter than what I was used to. I ended up using MSI afterburner to set a custom profile so that at specific temps, the fans kick into various levels. After doing that, the card does not seem to even hit 70 degrees celsius. Not the ideal solution, but I have afterburner start up with my system, and it simply runs in the system tray where I don't usually see it.