Hi @pddco
Yes, the router and the connector's been reset and I've tried the IP address 'trick' that @TimCurtis suggested - still no joy. I'm starting to think @licquorice is right about the 2nd port being there for show.
It would be interesting to know if anyone on here has ever had two PC's working off a single connector.
I'm going to see what happens if I plug in another connector for the 2nd PC.
Which PC didn't connect? Was it the one where you set the IP address?
If none of this works then the simple solution is a 5-port switch - preferably TP-Link.