HDMI on Linux Mint 19.3

Hi, I bought a new "Eye-Care" on HDMI monitor, Benq 28" HDMI / DP-1. The monitor works with the DP-1 cable attached but not with the HDMI cable attached. In settings, DP-1 shows up as red and HDMI shows as green when both cables are fitted. The problem is is that it only works through the DP-1 cable. I get a blank screen if I remove the DP-1 cable. Am I missing something or perhaps Linux Mint 19.3 doesn't support HDMI? Any assistance will be helpful, thanks.

Gael