Connecting a second imac screen - 21.5 inch late 2015 model to a new 2019 21.5 inch model

I'm trying to connect my old 2015 21.5inch imac as a second screen to my new 2019 21.5 inch imac. I've read that the one that is a display needs to be on an older operating system and have restored it back and connected the two via the thunderport and still can't get the target display mode to work. I've tried different keyboards and with and without the cmd-fn-f2 combination, still no luck :(

Any help will be greatly appreciated!