replacement graphic card problems

Greetings

I had a system built in Jan 2012 which included an Asus P9X79 mobo and
3 Geforce 570 video cards where I've been running a variant of Debian,
usually testing, ever since. I ran using 2 of those cards utilizing
first fully proprietary drivers and then a couple years ago I shifted
to the debian version of the same. This last spring 2 of the 3 cards
failed and nvidia retired support for the cards in their current
drivers so I went to using nouveau for my graphic card driver.
I purchased a Radeon RX570 graphics card and a week ago swapped out
Nvidia 570 card. I was unable to even get the system to post on first
try (I had all 4 monitors plugged into the gpu). Went back to the
Nvidia card and the system is working just fine. Tried the Radeon
RX570 again this time only plugging in 1 monitor - - - - the gpu light
do come on but I can't tell if much further is happening. I called my
purchase point and they requested I send it back to them. They are now
telling me that the card is working well on their M$ test machine.

I'm not sure what to do or what I did wrong in my install attempt.

Any ideas?

TIA
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