Help identifying mini display port on mid 2011 iMac please

I know you're going to tell me it's better to get a new monitor, but I'm a student on a very limited budget.

A relative gifted me a mid-2011 iMac 27 (running el Capitan atm) and it's great for all my student work.

My parents bought me a 2020 Macbook Pro 13-inch last year (running monterey) which is awesome, but now with my graphic design, I need a larger screen.

I read on Apple spec my mid-2011 iMac has a mini display port and is capable of target display mode, so I bought the adapter and cable from Apple UK (thunderbolt 3 (USB c) to thunderbolt 2 adapter and thunderbolt cable).

Thanks

When I connect to my iMac and follow apple's advice to enter target display mode, nothing happens. I press command and F2 (also tried Command - fn - F2)

I've set the keyboard prefs for the top keys to work as Fn keys.

Could someone explain what I'm doing wrong, please? Do I need a different cable?

Thanks -