Do you only have onboard Intel Graphics, without an add-in (AMD or NVidia) video card? If you have an add-in video card, the monitor(s) should be connected there. Check out the Inspiron 3881 photos here with the onboard ports identified on the back panel, and the location where an add-in video card would be on the back.  I suspect you're talking about the DisplayPort (DP) port. You could get a DP>HDMI adapter for you second monitor. Just make sure it's DP(PC)>HDMI(monitor) because adapters typically only work in one direction. You can't just use them in either direction and expect them to work...
Hi  The Inspiron 3881 has 1 HDMI 1 VGA and 1 unknow output at the back, what is this unknow socket?  At the moment I connected my monitor through HDMI output. I know I can connect a second monitor through VGA output. My second monitor has HDMI output can I connect it to my Inspiron 3881 rather than through VGA?   Thank you
Inspiron 3881 Monitors connection