using a late 2012 21.5" iMac as an external display

I've just got a new 2020 27" iMac and want to use my old 21.5" late 2012 iMac as an additional display. The old iMac is currently running El Capitan. I've connected them together using a thunderbolt 2 cable with thunderbolt 2 to 3 adaptor.

I've pressed cmd+f2 on the old machine, and nothing,

I've tried to get the new machine to detect a display - nothing

I've re-booted both machines - nothing

I've disconnected the cables and put them back in - nothing

Has anyone got any good ideas on how I can get this to work?

Thanks