Strange LAN network problem - can't "see" just one address on LAN

[Chris Green]

I have a small home LAN with a number of systems running xubuntu 21.10
and Raspberry Pi OS.  It's a pretty standard setup with the VDSL
router at 192.168.1.1, a few systems with fixed/static IP addresses
and then the rest assigned by the DHCP server.

My desktop system has for many years had the fixed IP 192.168.1.3 as
it's a mail server and ssh server so the router firewall has to port
forward to it.

Recently I have been trying out some changes to how the DHCP/DNS is
managed and, for some strange reason, my desktop machine at
192.168.1.3 could no longer 'see' the router at 192.168.1.1.
Everything else works perfectly still, my desktop machine can see and
connect to (via ssh etc.) other systems on the LAN and other systems
can connect to my desktop machine.  Also all other systems on the LAN
*can* connect to the router at 192.168.1.1.

Simply changing the IP address of my desktop machine from 192.168.1.3
to 192.168.1.245 (above the DHCP server's range) fixes the problem.
(I have to change the port forwarding in the router too of course).

So can anyone suggest why this might be happening and/or ways to
diagnose what the issue might be?  I'm completely stumped at the
moment. 

The desktop runs Network Manager, I've changed IP back and forth with
the Network Manager panel applet and by editing the file
/etc/NetworkManager/system-connections/Wired connection 1.nmconnection
directly.  Exactly the same happens either way.

Help! :-)

-- 
Chris Green