"I plugged a monitor in through the motherboard and checked the device manager and it doesn't seem to be there, even after installing the drivers."
Since the on-board graphics adapter is still usable with the Radeon card installed, that suggests the on-board adapter hasn't been disabled in the BIOS settings. It's usually disabled automatically when an alternative graphics card is detected in one of the PCI/PCI-Express slots, but on your PC it may have to be disabled manually.
Plug the monitor into the Radeon card, restart and go into BIOS Setup
Disable on-board video, then be sure to "Save to CMOS and Exit (F10).
The Radeon card should then work.