2012 27" iMac in Target Display Mode as a second monitor

I have a 2012 27" iMac running High Sierra 10.13.16 that I am trying to use as a second, extended monitor for my Dell laptop from work.

Using CMD+F2 and CMD+FN+F2 does not put the iMac into Target Display Mode (TDM)

I also have a USB-C to Thunderbolt 2 adapter and Thunderbolt 2 cable that were purchased from Apple as the Dell has a USB-C, VGA & HDMI ports.

Is it possible to use the VGA &/or HDMI ports on the Dell and use that to connect to the iMac Thunderbolt port?

Is there anyway to use the iMac as a monitor WITHOUT using TDM?