My secondary (external) display wont come on

My Mac mini screen won't turn on - Apple Community

very similar issue.

So I just got the mac mini yesterday. I first installed it on one monitor, because the mac didn't come with any cables... so I had to buy the HDMI (2.1) -> USB-C.

I have 2 Acer x34 predator monitors, that were connected to different PCs over the 6 years, without any problems.

Once I connected the second display from HDMI to usb-C the system recognizes it correctly and I am able to set the resolutions and refresh rates and all... BUT, the display comes on for 2s and then turns off. The system still sees it as it was always connected (and it is). Tried different ports, tried restarting, tried messing with monitor settings, etc... always the same.

The cable is brand new. and is supposed to be capable of 8K.

So is not the cable issue, is the system issue.

Also for some obscure reason, I can only set the displays to 50hz... independently if it is HDMI or USB-C.

if I connect the second monitor via HDMI then it works... and the first one doesn't