Target display troubleshooting

Hi All,

I'm struggling here to figure out why target display mode is not working on my iMac and MacBook Pro. I've gone through the suggestions "If Command-F2 doesn't work" and no dice. Hoping maybe I just overlooked something or any suggestions you all have. Here's what I'm working with:

iMac (21.5-inch, Mid 2010), MacOS Sierra 10.12.6

MacBook Pro (Retina, 15-inch, Mid 2015) MacOS High Sierra 10.13

Connecting the two via Mini DisplayPort to Mini DisplayPort cable(from amazon). Tried an Apple Thunderbolt cable I had laying around and it also doesn't work.

Using Keyboard Viewer I confirmed Cmd. F2 is being pressed. Also tried Cmd. fn. F2 for good measure.

Restarted both machines and reset the NVRAM on the iMac.

Tried pressing Cmd F2 while logged in and logged out of profiles.

Not sure what else to try other than buying yet another cable but I would really love to get this up and running.

Thanks in Advance!