Have you plugged the monitor in to the graphic card video plug?
The computer will only use one or the other, unless you set it up in crossfire, which you would not do in your case since the r270 is a lot more powerful than the apu graphics.
Perhaps someone can chime in with any specific settings you may need to use.
When I switch from on board to a card , changing the plug to the correct device and selecting "use this device" was all I needed to do, your case may be slightly different.