Did you follow the steps to set it as the default X display? Does your Pi "startx" automatically?
If your 2.8" display is staying solid black, after showing the boot up logo, then it should be working. Plug in an HDMI display and follow the final step to test it:
cat /dev/urandom > /dev/fb1
If this works, and you see random rainbow noise on your DFRobot display- then congrats, it's setup successfully!
To keep everyone's sanity, I didn't include steps for disabling HDMI output and switching only to the USB display, since it would be a nightmare to debug such a setup if it didn't go well.