A server could not be found. Download and install Plex Media Server, and it will appear here

Help!!
Just can’t understand it. My router ASUS DSL-AC68U died and has been replaced on warranty. I’ve configured my NAS with the same IP address as before. I’ve downloaded & installed the latest Plex, stopped and started it numerous times, but I’m still getting the “could not be found” message in the browser (and any other devices).

I note that there is nothing in the homes/plex share, not that I ever checked before, so I’ve no idea what might normally be there. the plex account has full control to the share.

Is this a security thing? a router firewall thing? Any ideas?

Thanks