P2418D, Radeon HD 6450 HDMI out, 2560x1440 not supported

I'm afraid max over HDMI supported by your AMD HD6450-based card is 1920x1200: https://www.amd.com/en-us/products/graphics/desktop/6000/6450#

2048x1080 is actually a lower resolution than 1920x1200 in terms of pixels/bandwidth. To get the full 2560x1440 from your monitor, try a dual-link DVI-to-HDMI adapter.

The P2418D monitor on a hardware level supports 2560x1440 via its DP and HDMI in ports. See pages 8, 14, 15, 47 of the User's Guide. So the fault has to be the video card or its drivers, not the P2418D.

As swamped stated, the low end AMD Radeon HD 6450 HDMI out port maximum resolution is 1920x1200. I cannot find any adapter that can convert from HDMI to DVI at 2560x1440. All I can find only do 1920x1080. My advice, get a better video card with a full size DP out port.

There is no such thing as a dual-link DVI-to-HDMI (passive) adapter because HDMI is essentially a single link interface. Though there existed such thing as Dual-Link HDMI interface theoretically, it never was produced in practice by any GPU or monitor manufacturer.

The simplest and correct way to fix this issue is to use a DVI-D Dual-Link cable together with a DVI-D Dual-Link capable monitor without any converters or adapters or change the card for the one with DP output. Though your current card might be in fact able to output 2560x1440@60Hz through HDMI interface, it's likely this is limited by the driver as it is a low-end graphics card and cards of this class and that generation usually had HDMI module limited at the pixel clock rates of HDMI 1.0 or 1.2(a) standards and that's approximately FullHD @60hz max or in terms of pixel clock rates it's either 165MHz or 225MHz.

@swamped207  I've seen adapters that will output dual-link DVI from a DisplayPort or USB-C source, but not from HDMI.  I also haven't seen an adapter that will accept a dual-link DVI signal and output HDMI 1440p, even though I realize that's not what's called for here.

@strahinja  It appear your GPU's HDMI output does not support HDMI 1.3, which is necessary for 1440p 60 Hz output.  So your options would be to replace the GPU or get a USB adapter that relies on "indirect display" technology such as DisplayLink -- not to be confused with DisplayPort.  However, DisplayLink comes with some drawbacks that can be significant in some use cases, and given what the adapters cost, in your case it might actually be less expensive to buy a modern low-end GPU that will support 1440p.  DisplayLink dongles are often used to connect more external displays than are supported by the source system's available output connectors and/or GPU's max display limit, since DisplayLink displays don't count toward the latter.