Thunderbolt 3 - Thunderbolt 2 problem
On the Presonus forum it appears I am not the only one with this connection problem. There are very many unhappy un-users on there...

I uninstalled and reinstalled their Universal Control software several times and tried different approaches. One thing I did discover is it is very important to open the Security and Privacy section in Mac system preferences and change the settings to allow third party software. 

I also followed the advice of a contributer on the Presonus forum, advice which is not mentioned in the Presonus setup information... 
Following an install of the Universal Control software you switch off the Quantum and unplug the TB cable. You reboot, reconnect the TB cable and switch on the Quantum and it worked. Thanks for the advice and happy recording.