In Device Manager are there any Yellow Exclamation marks under "Display" or Error messages when connected with HDMI cables to HDMI Monitor?
Have you tried a different HDMI Cable in case it is defective?
Disconnect all Video Outputs cables except one HDMI cable connected to your HDMI Monitor from your GPU card. Does it work now?
Here some websites on how to fix HDMI Outputs not working . They are all similar:
https://troubleshooter.xyz/wiki/fix-hdmi-port-not-working/
https://www.thewindowsclub.com/hdmi-port-not-working-properly
4 Ways to Fix HDMI Ports Not Working in Windows 10 | Driver Talent
Also try this:
How do I enable my HDMI port on my computer?
Plug the other side of the cable into the "HDMI IN" port on your TV or monitor. Right-click the "Volume" icon on the Windows taskbar, select "Sounds" and choose the "Playback" tab. Click the "Digital Output Device (HDMI)" option and click "Apply" to turn on the audio and video functions for the HDMI port.