Folks,
Finally got a configuration from O365 which works, lots of the other changes from Microsoft but this is only one which works and is a perminate configureation change in O365.
Within O365 Admin, Go to Security > Polices & rules > Threat policies > Anti Spam
Add a IP Address to connection filter policy (default) and click edit connection filter policy in the “Always allow messages from the following IP addresses or address range” field/textbox add the PLEX IP address of the server 192.254.122.79, (press enter) to force the webpage to accept the IP Address.
See my attached screenshot.
