Monitor not working connected to the motherboard?

the port on the mobo is connected to the integrated graphics which is turned of because you installed a dedicated card, and theres no way to make the card use it, besides the fact that it takes a certain piece of hardware per monitor, so each graphics card has a limit on how many monitors it can support, normally its 1 (even with 2 ports), you have one that can (suprisingly/luckly) use both ports at the same time, but its not gonna have support for a 3rd monitor, if its a newer card you might be able to get another one of the same card and hook them together (dual card setups are somewhat common), and then use up to 4.

You need to check the cables of the second monitor if this second monitor is not faulty.Interchange monitors to check if it is faulty.check connection pins also.Sometimes pins get damaged during improper installation.

Assuming you have correctly installed the video card in the correct socket type (PCI-Express x16 for example), then you need to make sure of two things: -Is your graphics card able to support 3 monitors? Some cannot. Consult your manufacturer's website to make sure. Regardless, updating your graphics drivers to the latest version is a good idea. -Have you fully and properly connected all the monitors? Some monitors can be a bit tricky when it comes to being fully connected. Mixing VGA and HDMI isn't exactly the best thing to do since one is a better quality connection than the other. But if you don't care about quality, it will do. And of course this is all assuming your motherboard and operating system support it. Check the manufacturer's page to make sure that it supports such graphics cards and associated technology.