iMac 21.5 inch, late 2009 on High Sierra 10.13.6 as a 2nd monitor

Hello!

I recently got an iMac (Retina 5k, 27 inch, late 2015) running on Catalina 10.15.6.

I'm hoping to use my old iMac (21.5 inch, late 2009) running on High Sierra 10.13.6 as a 2nd monitor.

I've tried thunderbolt 2 to firewire, I've tried a mini-display port cable, even a thunderbolt 2 to ethernet, or just an ethernet cable. Nothing is working. Using command+F2 does nothing, I tried holding fn too (even though I don't have that setting checked in system preferences), I've restarted, unplugged, replugged in, logged out, even tried using different keyboards (all mac keyboards) and nothing is working.

Is Target Display Mode not possible on the 2009 iMac because it's 21.5 instead of 27 inches?

or am I missing something else?

please help!