Use 2015 iMac as secondary display for 2019 iMac 5k

Hi,

I am trying to connect my old 2015 iMac (21.5") to be used as a secondary display for my 2019 iMac (27") I have a thunderbolt 2 cable connected to a thunderbolt3 to thunderbolt 2 adaptor to connect them both.

When I look at the thunderbolt bus I can see the both devices are connected but it doesn't pick it up as as second screen. I've tried target display mode (CMD + F2) and. I have tried holding down option on the display screen in system preferences (detect displays) to force the devices to be found. but still no luck. Also as my older Mac is only 21.5" it doesn't support sidecar although it is running Mohave) so cannot use this feature

Any help would be much appreciated

cheers