Cannot connect Imac as external display

dear Community, I'm trying to use my old Imac as an external monitor for my new mac book pro. It actually worked around two years ago so I suspect it must be relating to ios change but I need your expertise to check if I let it go or get the change to recover it.

My Mac book pro : 14i, 2021, M1 with last version of Monterey

is connected with a USBC adapter to thunderbolt plugged in the Imac (I assume the cable is right cause it used to work in the past)

IMac : 27i,mid 2011 with the last version of High Sierra

The Imac is not detected in the monitor preferences of Monterey

When I use the previous working sequence on the Imac keyboard cmd+F2, I get nothing on the imac screen.

Any insight please?

Thanks

Christophe / France