Hello, I have an AMD R9 390 Series, and I recently just bought a MSI Optix G27C5. It is able to go up to 144hz with only an HDMI and 165hz with a displayport. For now, i'm just using an HDMI, but when I go to my display settings to change my refresh rate, the highest option is 60hz. https://prnt.sc/wa2f49
is your cable a HDMI 2.0? if its an older cable it doesnt support 144hz over HDMI
did you enabled highest HDMI version in your monitor? (Menu in Monitor)
did you installed the driver for your GPU?
1. disable Win10 auto-driver-install
2. install latest AMD driver
test again ;)