Agent won't connect to gateway

I had an agent that wouldn't connect to its gateway server. I tried all the tricks but no joy. This was the event that wouldn't go away (which is very common by the way):

Log: Operations Manager
Source: OpsMgr Connector
Level: Error
Description: The OpsMgr Connector could not connect to NAME:5723. The error code is 10060L(A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.). Please verify there is network connectivity, the server is running and has registered it's listening port, and there are no firewalls blocking traffic to the destination.

This caught me out. It turned out to be a Trellix firewall component blocking 5723. As soon as it was removed it connected.