The problem turned ou to be the SATA to MOLEX cables that were a bit too bented, and therefore couldn't power the risers !
I put them in a better position and the cards were finally detected.
Now, I flashed the BIOS of the cards (with Polaris and ATIflash), but the fans don't spin anymore (only a bit when I turn on the MB, but when W10 boots,
Glad to hear you fixed the previous problem.
RX series mostly like that when idle, and will start spinning its fans when reaching specific temps. But in your case, you said "a bit warm" how you know it? Touching? How about temps on software view?
You can try to set your card fan to manual with a specific fan speed, check it Its fan spinning or not.
All about bios, it's clear enough above.