Can't get correct resolution on UltraWide LG Monitor with Mac Mini

The info in your link states:

• Supports video resolutions up to up to 1920x1200 and 1080p (Full HD); easy-grip treads; low-profile design

He needs something like this:

https://www.amazon.com/gp/product/B00PJ3LSIG/ref=ppx_yo_dt_b_asin_title_o00__o00_s00?ie=UTF8&psc=1

Supports video resolutions up to 3840x2160 (Ultra HD) at 60 Hz

There is the Display Menu app in the Mac App store that allows more resolutions. It is free for basic app but there are in-app purchases.

‎Display Menu on the Mac App Store

What specific mini displayport>HDMI cable are you using? (mini displayport video output is a subset of the Thunderbolt 2 signal). A link would be nice. Not all converter cables support higher than 1080p. I was playing around with a displayport>HDMI cable (displayport and mini displayport are the same except for connector geometry) and could only get max of 1080P on a 4K monitor.

that HDMI was not meant to be used with computers.

Well, that needs to be explained a bit.

We have a 4K OLED TV for our network, TiVo and Blu-ray viewing. With all connectors between the set-top boxes and the TV being HDMI, it's rather obvious HDMI can handle 4K (and more).

The big "but" here is back in (Mountain Lion I think it was), Apple decided to treat HDMI as what it's intended for. As a TV signal. Before that, you could use HDMI just as readily as DisplayPort and get all of the resolutions your video hardware and monitor supported. Then all of a sudden, you couldn't. All it took was upgrading the OS to throw things out of whack. Same hardware you were using before, but you found yourself limited to 1920 x 1080.

Apple's thinking was to use HDMI as a replacement for expensive break out boxes for video professionals. You'd use DisplayPort for your RGB signal computer monitor (where all of your video software interface was), and HDMI to feed the working video in TV's ycbcr color space to a separate TV set. That way, you see the final color as it will appear on an actual TV in the proper color rendition, and not hope the RGB representation was close enough.

A lot does depend on the monitor. Some folks can't get HDMI to use a monitor's maximum resolution for anything. Other monitor's work just fine with an HDMI connection. I suppose it mostly depends on the circuitry of the monitor if it will negotiate its maximum resolution with your computer or not.

The cable is the reason. HDMI is meant for TV resolution and color. The cable is at best, confusing both the Mac and monitor.

I see this monitor only has HDMI. I'd return it and get a monitor meant to be used with a computer.