Target display mode not working on 21.5 Mid 2011 iMac

Hi all,

So from my research I believe that a 21.5" Mid 2011 iMac running High Sierra 10.13.6 should be able to connect via TDM to my Macbook Pro (2018, Catalina 10.15.4).

I have an apple thunderbolt 3 to thunderbolt 2 connector and an off brand thunderbolt 2 cable.

The 'Use F keys as standard function keys' is unchecked.

However Command +F2 on the iMac does not connect the two.

Any tips on where I am going wrong here?

Help much appreciated!