I've read a couple times on this forum that you need to have the 2nd graphics card detect that it is being in use for it to function.  That requires either plugging in a 2nd monitor, or attaching a dummy plug that tricks the card into thinking it's being used.  I've found one here:

https://cablesaurus.com/index.php?main_page=product_info&cPath=2&products_id=13&zenid=e4d7820ab94335667e21685e7a001f28

And they even charge in USD or BTC.  I hope this helps!