Mac Mini 2011 QHD (1440p) resolution

Hello folks,

Looking for an answer as my local Apple Support wasn't able to find a solution.

I have a Mac Mini (2011 model) that supports up to 2560×1600 resolution through the Thunderbolt port, and 1920×1080 through the HDMI port (source). I also have a DELL display with QHD (2560×1440) resolution, and also a HDMI cable supporting this resolution (tested with a MacBook Pro on the same display).

The first curious thing is that I can surpass the officially supported 1080p with HDMI, and can reach a resolution of 2048×1152.

Now here comes the problem. I want to connect the Mac Mini to the display with support for the full QHD resolution. Since the Thunderbolt port is compatible with the Mini DisplayPort standard, I used a thrid-party Mini DisplayPort—HDMI adapter with the HDMI cable, but the resolution remained the same 2048×1152.

Since both the display and the HDMI cable support the desired resolution, the culprit is either the Mac Mini or the adapter. I've read on a forum, that I might need to buy an active adapter. I've also checked the Belkin 4K Mini DisplayPort—HDMI adapter from Apple's website, which might or might not work with my model. I can't tell, and neither could Apple.

I'm stuck without a solution. Does anyone have any advice or relevant experience regarding this? I really want that shiny QHD resolution.

Thanks!