External display issues on 2019 iMac

Hi all. I have an iMac 27” 2019 running Sequoia which won’t recognise an external display plugged into the Thunderbolt ports via an HDMI adaptor.

I’ve checked the ports on the iMac by plugging an external HDD in and the computer sees that ok.

I’ve checked the external monitor, HDMI cable and adaptor by plugging it into my 2020 13” MacBook Pro also running Sequoia, and it recognises the display immediately.

I’ve tried trashing the displays plist file, resetting PRAM, restarting etc. but no joy.

It’s worth mentioning that adaptor is not an Apple one so I suspected that at one point but it’s strange that it works absolutely fine with my MacBook. The only difference I can see is that the Firmware version for Thunderbolt is different on the MacBook than the iMac but there’s no option to upgrade in the system report.

I’m sure it’s something really simple and obvious but I’m at a loss at this point so would appreciate any ideas!