I think you meant I couldn't find the setting in the bios.
It is possible that there is none, oem bios tend to be very restrictive and only support the purchased configuration.
If you attach a monitor directly to the motherboard, does it work?
What then are your options?
1. The usb dongle idea from 13thmonkey looks reasonable to me.
I have not used one, so I have no idea if there are any issues.
2. You do have a second pcie x16 slot into which you could insert a second graphics card.
I would not be comfortable with that unless you upgraded the psu.
3. You could replace your card with one that has 4 monitor support like a GTX1060
https://www.newegg.com/evga-geforce-gtx-1060-03g-p4-6160-kr/p/N82E16814487263
It has one dvi, one hdmi, and 3 displayports so you are going th have to scramble for adapters.
This will also need a psu upgrade.
I think I might experiment with one of the USB dongles and see how you do.