Eth one way only

Hi All

I have the following connected on Ethernet to a Netgear router.
Wifi disabled on all, no Internet connection.
Using ConnMan the IP’s are set manually with NetMask 255.255.255.0
and Gateway 192.168.0.1
All settings confirmed with lsconfig.

Desktop Debian 10 on 192.168.0.2
BBB Debian Stretch on 192.168.0.4
Laptop Mint 19 on 192.168.0.6

Testing the connections with Ping:
From Desktop 192.168.0.1 OK (Router)
with Debian 10 192.168.0.2 OK (self)
192.168.0.4 BBB Unreachable
192.168.0.6 OK

From Laptop 192.168.0.1 OK (Router)
with Mint 19 192.168.0.2 OK
192.168.0.4 OK
192.168.0.6 OK (self)

From BBB 192.168.0.1 OK (Router)
with Debian 9 192.168.0.2 OK
192.168.0.4 OK (self)
192.168.0.6 OK

I have tried so many thing that I have lost track.
The idea is to do the development on the Desktop, send the files to BBB and compile.
So from the above it seems that Debian 10 can ping everything except the BBB, yet responds
to pings from the BBB and others.

Would love to hear some suggestions on how to solve this problem.

Regards