Where are you setting the resolution? A PC doesn't just automatically go to the highest native resolution when a monitor is plugged in. You need to set the resolution in Windows for example under Settings, or via display adapter Properties > List all modes. If you have some sort of super scaling enabled, or custom resolution set in Adrenaline, that can also affect refresh rate and resolution. If you are using multiple monitors, are they all set to the same refresh rate and resolution? Are you using an HDMI cable, or a proper Display Port cable for the new monitor? Use DP, rather than HDMI if you are.