Now find another video cable and connect it to the AMD GPU and then to another available port on the monitor. In other words, while macOS is running, hot-connect another cable:
- You have one HDMI cable already connected between on-board HDMI port and monitor.
- Connect another video cable between the AMD GPU and the same monitor (or a different monitor).
If connecting two video cables to the same monitor, just change the monitor input -- flip back and forth between the two inputs. Are both inputs working?
If connecting two monitors, do both monitors show the desktop?