So it's possible the ports on the monitor could have taken a hit but back to the new GPU. I don't see the make and model so I can't check if it supports your setup. HDMI LICENSING costs per port so many cards only enable one at a time so try the Mini-DP connection all on it's own to a DisplayPort on a monitor to test it out.

Also, drivers. Sometimes I have to use DDU (see google on what and how) to remove old drivers, then I can install the graphic cards drivers per the maker's suggestion. Usually that would be from the maker's site or from AMD or Nvidia directly.