New iMac - difficulty connecting to my Cintiq 22HD

Due to ongoing problems with my old iMac, Apple replaced it with one of the new machines that have just been released. My problem is that I had to buy a new Connector for my Cintiq (DVI to USB-C). The problem is that the iMac cannot see the connector once it is inserted. I have installed the latest drivers. I have spoken to Apple and Wacom support. Wacom confirms that I have the correct connector but it looks like it hasn't been tested on the very new iMacs. Apple support suspected some kind of incompatability issue between thunderbolt 2&3. I am still working with them to work out a solution but I wondered if anyone else had any insight or solutions?

Any helpful suggestions much appreciated.