iMac 21,5" - as extermal monitor

Hi,

i have iMac 21,5" (year 2015) running BigSur MacOs and I want to use it as external monitor.

Before, i had MacBook Pro 13" (2018) and I could use my iMac as external monitor via Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter once I pressed command+f2 keys.

But in October I upgraded from MacBook Pro 13" to MacBook Pro 14" (2021).

And with the new laptop this is not working anymore. I connected iMac and new MacBook Pro and with pressing on command+f2 nothing happens.

Is it possible, that this feature is not working on M1 Pro procesors or am I missing something. Is there any other solutions in using iMac as external monitor for M1 Pro processor Macs?

Thanks!