Cannot connect to Plex on my PR4100 anymore

I have had this system working fine for ages, never been a problem since I installed it.
But at the weekend I had to reboot my router and since then I have not been able to get any of my Plex players to see the server.
This is two Android apps, a Windows 10 PC and a Macbook.
If I try to access the app from the WD control panel I get this error:

This site can’t be reached refused to connect.

Have turned off firewall, makes no difference.
Rebooted the server and all connected devices.
A bit stuck now - any clues on what to do next?
Thanks in advance.

Oh and the IP address of server is the same as it was, although it shouldnt matter with DHC turned on.
Manually installed latest firmware and software updates for Plex and WD

Try uninstalling and re-installing Plex.

I had done that, but it still wwould not work. I just went throught this procedure though, which I thought I had done…

Select Plex Media Server and turn OFF Plex

Reboot the My Cloud.
Log in to the My Cloud Dashboard again.

Select the Apps tab again.
Select Plex Media Server and turn ON Plex.

Click the Configure button to access Plex in the browser window.

Now its on again. Good!