Mac doesn't detect external monitor odyssey g9 after after updating to macOS Tahoe 26.2

hdmi

Is that HDMI port on MacBook to HDMI port on monitor?

I don't think this would matter, but if it is Apple Silicon, check System Settings > Privacy & Security > Security section. Set Accessories to always ask, then connect the monitor. If it asks, then allow it and you can change that setting back to whatever you want.

What ports and what cable type?

What video protocol does the monitor expect?