Latitude E7470 don't have maximum resolution for 2560x1440 external monitor

If you want a bit more technical background/history, here's why this happens.  DisplayPort has always supported higher bandwidth than HDMI at any given time, or has at least implemented the higher bandwidth support first.  So for example, once DisplayPort offered enough bandwidth to support 2560x1440 @ 60 Hz, display manufacturers didn't want to wait until HDMI caught up before releasing their 2560x1440 displays.  Instead, they simply added an HDMI input that only supported 1080p @ 60 Hz.  And for some consumers, that was fine.  For example, if you wanted to connect your PC via DisplayPort and a game console via HDMI, and the game console only supported 1080p anyway, then this limitation didn't matter.  And later on, once HDMI was updated to support 2560x1440 @ 60 Hz, newer displays implemented the newer HDMI standard and therefore could accept that resolution on both DisplayPort and HDMI (although even then, some manufacturers might have kept the old HDMI chipsets to cut costs.)

This is happening again with 4K.  DisplayPort has supported enough bandwidth to run 4K @ 60 Hz for years, actually since before 4K displays even existed, but HDMI has only recently caught up.  As a result, the early 4K displays on the market only accepted 4K @ 60 Hz on their DisplayPort input and were limited to 1080p or maybe 1440p @ 60 Hz over HDMI (or 4K @ 24 Hz over HDMI, which might be useful if you're hooking up an Ultra Blu-ray player).  Now that HDMI has been updated, some newer 4K displays can accept 4K @ 60 Hz over HDMI.  However, it's also worth noting that the HDMI outputs on many laptops won't do 4K @ 60 Hz, including many or maybe even all current Dell laptops, so that can become a problem too.  Both the source output and the display input (and the cable you're using) need to support the bandwidth required for the resolution and refresh rate you're trying to use.  Color depth is technically also a factor for bandwidth, and although that hasn't really mattered before for displays, it's starting to matter now because of HDR. DisplayPort 1.2 (the most common standard out today, especially since it's what Thunderbolt 3 uses) has enough bandwidth for 4K @ 60 Hz, but not 4K @ 60 Hz with HDR, for example.  For that, you need DisplayPort 1.3 or newer, but that's still relatively rare even on desktop GPUs and practically non-existent on laptops, even though there's already a newer DisplayPort 1.4 standard now.  On the HDMI side, HDMI 1.4 can support 4K @ 24 Hz as I mentioned earlier, and HDMI 2.0 can support 4K @ 60 Hz or 4K @ 24 Hz with HDR, but not 4K @ 60 Hz with HDR like DisplayPort can.  HDMI 2.1 will support that, but it hasn't arrived yet.